In the 1st through 5th centuries, most of southeast Asia was dominated by Funan, which included most of present Thailand, Laos, Cambodia and the Mekong delta region of Vietnam. Funan was supplanted around the 6th century by Chenla. Annam was a small coastal strip in north Vietnam and southern China, and south of it Champa controlled a coastal strip in central Vietnam.
Pressure from China pushed Annam southwards, and in turn Annam pushed Champa south to roughly modern central South Vietnam. Around the 12th century the Khmers emerged as a great empire controlling all the lower Mekong, leaving Sukhothai as a small version of modern Siam and Laos. The Khmer Empire collapsed after Siam seized Angkor in 1431.
The Khmers (Cambodia) thereafter found themselves pinched between their more powerful neighbours Siam and Annam. In the late 18th and early 19th century, much of Cambodia was partitioned between Siam and Annam. In 1844 what was left of Cambodia became a protectorate of Siam.
Around the 15th-16th century the Kingdom of Luang Prabang emerged along the upper and middle Mekong River. Vientiane and the middle Mekong broke away in 1707. Both Luang Prabang and Vientiane came under Siamese suzerainty in 1778. During most of its existencae, Laos (Luang Prabang and Vientiane) remained disputed between Siam and Annam. Laos did not re-emerge as a distinct and separate entity until the French declared a protectorate over this disputed region in 1893.
Annam expanded further southward into Champa in the 16th-17th centuries, and into the Mekong Delta in the 18th century. Annam (with capital at Hue) seized Saigon in 1776. I'm not sure when, but north Annam and the Mekong delta region became separate entities as Tongking and Cochinchina respectively. Annam reunited the whole region (equivalent to modern Vietnam) with the creation of the Vietnamese Empire in 1802, but Cochin, Annam, and Tongking remained separate administrative regions.
France annexed Cochin in 1862 and 1867, with their capital at Saigon. Cambodia became a French protectorate in 1863, and Siam gave up its claims to the area in 1867. The French then made protectorates of Annam and Tongking in 1884, and united the whole lot into the Union Indochinoise in 1887 (with the capital still at Saigon). Laos joined this union when it became a French protectorate in 1893. After a confrontation with France, Siam ceded large chunks of its territory which France patched on to northern Cambodia and Laos in 1907. T.F. Mills, 19 October 1997
Until the Khmer Cochin region (greater Mekong delta) was annexed by Annam (in 1698 and 1731), Annam was actually known as Cochin to Europeans. Cochin was a Portuguese corruption of "Ko-chen," whose meaning is unknown. -China was tacked on to Cochin to distinguish it from Cochin in India.
"Indochina" as a name was proposed for the region in the early 19th century by Scottish poet and orientalist John Leyden, because it lay between India and China -- and perhaps because it had in its early history been dominated by those superpowers.
Here are some more notes on the formation of the region:
Tonkin (dong kinh = eastern capital) was the original Annamese state (capital at Hanoi) in 939, and remained subject to China until independence in 1428. Annam (an nam = peace of the south) was dynastically divided into Tonkin, Annam and Cochin in the 16th century and reunited in 1802.
Funan and Champa were Indian states. Champa was founded in 192 and largely annexed by Annam in 1472.
After WWII Cochinchina was briefly re-formed as a French Overseas Territory (1946-49) before joining Vietnam. "Viet" was the name of an old Chinese principality. (So, South Vietnam was "south Viet south," but that wasn't actually its official name.)
T.F. Mills, 20 October 1997
The Vietnamese people were originally in the Red River delta of what is now Vietnam. During the period of 111 B.C. and 928 A.D. the Vietnamese people were ruled by the Chinese who had named the area An Nam or Annam, meaning peaceful south. Independence was regained after a long period of struggle during which tactics were developed that would be repeated in succeeding centuries. In 939 Ngo Quyen became the first king of an independent Vietnam, which he ruled from Co Loa. His dynasty was overthrown in 968 to be replaced by Dinh Tien Hoang, who renamed the country Dai Co Viet (Great Viet), although the Chinese recognized it as a state within the Chinese empire (Giao Chi prefecture). When Dinh died, Le Hoan seized the throne and finally expelled the Chinese. He was succeeded by Ly Thai To, who move the capital to Dai La (Hanoi) and founded the Ly dynasty, and the country's name was changed again in 1054 to Dai Viet.
During the Ly dynasty the Vietnamese people began to migrate south into what is now Central Vietnam, an area controlled by the Cham.
The Ly dynasty was replaced in 1225 by the Tran dynasty, which successfully repelled three Mongol invasions and took Champa in 1312. Although Champa would reemerge in 1326, it would finally be defeated by the Vietnamese in 1471 and the Chams would retreat into Khmer. In 1407, the Chinese, under petitions from landowners threatened by land reforms under the Tran dynasty, retook Vietnam and held it until 1428, when the Le Loi defeated the Chinese army and established the Le dynasty which would rule until 1788.
The Le dynasty itself would grow weak and in 1527 the Mac family seized the throne, resulting in the Nguyen family doing the same in the south, which resulted in a partition of the country until 1802. Each family (and the Trinh family which followed) claimed the title of chua, or lord (or prince), ruling on behalf of the king. During this period, the Nguyen family conquered the Mekong region inhabited by the Khmer. Following a rebellion whereby the country was divided into three parts ruled by the three Nguyen brothers who had successfully defeated the Trinh family. The Le emperor fled to China and while the Chinese recognized him as King of Amman, they were defeated by Quang Trung (Nguyen Hue) who proclaimed himself emperor. In 1802, a period of unrest that began with the death of Quang (d. 1792) resulted in the reunification of Vietnam under the Nguyen dynasty.
In 1834, the Nguyen dynasty attempted to take Cambodia with the result that Cambodia was administered as a protectorate of both Vietnam and Siam. After the death of Viceroy Le Van Duyet in 1832, the emperor began a program of suppressing Christianity, which over time would lead to the French becoming involved in the region following even stricter sanctions under later emperors. In 1858, the French took Tourane (Da Nang) and and Gia Dinh (Saigon) in 1959. In 1861 provinces surrounding Saigon were captured and in the Treaty of Saigon (1862), Emperor Tu Duc ceded the provinces surrounding Saigon to the French, the region being renamed Cochinchina. Tu Duc's attempt to crush a Christian rebellion in Bac Bo (Tonkin) led to French getting further involved in storming Hanoi and the formal annexation of Cochinchina in 1874. In 1882, the French were forced to withdraw from Tonkin, but in 1883 they took Hue and a Treaty of Protectorate was signed establishing a protectorate over the northern and central regions of Vietnam, which the French would name Tonkin and Annam. It should be noted that the name of Annam appears not to have been well received by the Vietnamese as it was a reminder of the 1,000 years of Chinese occupation. (which is one of the reasons for such a long and convoluted history, especially as some material is hinted at or minimally addressed at FOTW, but brings into focus what is said there). Phil Nelson, 1 September 2003
Recently I came across an article entitled "The National Flag of Viet Nam: It's Origin and Legitimacy" by Nguyen Dinh Sai which provides some background and information on the flag of Viet Nam (or the flag of South Vietnam). The document is located at dienhanhvanhoaquocte.org and towards the end is highly political in its thesis. For those who want to download, the document is 2.84 MB in size.
The first flag illustrated in the text is the Long Tinh Ky flag - a yellow flag with a red circle in the center approximately half the height of the hoist. It is also shown with a blue fringe, but not the type of fringe we refer to today. The fringe represents dragon's scales and the ocean and therefore similar to what we may visualize this to be. (See article, p. 2). The dates ascribed to this flag are between 1802 and 1885 and used as a royal flag until 1863 when it became a national flag. In 1885, it is reputed to have become the flag of the resistance to French rule, at which time it was first documented.
The second flag shown is that of the Dai Nam, a yellow flag with variations of the Chinese letters Dai and Nam at 90 degrees counterclockwise to normal presentation. The difference between the Chinese letters and those of the flag are attributed either to poor calligraphy or bad representations by westerners who documented the flag. The flag use is noted between 1885-90. The flag is also shown at Worldstatesmen.org (as is the prior flag).
The third flag shown is the Dai Nam national flag from 1895-1920, similar to the last flag of South Vietnam. For both this and the previous flag, the cite is back to Worldstatesmen.org. Unfortunately Worldstatesmen.org does not cite it's source.
The next flag is the Long Tinh Ky, dates given from 1920 to 10 March 1945. This flag is documented by Jaume Olle, and most recently by Željko Heimer in his posting on 27 November, although Željko labels it as the Dragon Flag of Annam.
The fifth flag in the series is the flag of the French protectorate and shown by Željko with his post of the Dragon Flag of Annam from Flaggenbuch. Dates of use are given as 1923 to 10 March 1945.
On March 11, the article notes that the Japanese overthrew the French colonial government and returned to a version of the Long Tinh Ky flag with a darker yellow. Here I disagree with the date by one day as the Japanese overthrow appeared to take place on 9 March. During the Japanese occupation the article cites it as the imperial flag.
At the same time, the author reports the use of the Que Ly flag, similar to Santiago Dotor's Ly image on the South Vietnam page. The author attributes this flag as the national flag during the Japanese occupation.
I'll bypass describing the flags of North Vietnam and the Viet Cong.
And now to address the flag of the Republic of South Viet Nam 1946–1948 as the author describes it. This is the flag we show, above as the flag of Indochina, or close to it in the fact the author shows white stripes in place of the yellow between the blue. The author attributes the blue stripes to the three components. Reported period of use: 1 June 1946–2 June 1948. Phil Nelson, 13 December 2004
image by Jaume Ollé and Tomislav Todorovic, 9 September 2014
Republic of South Vietnam was founded in 1946 as the Autonomous Republic of Cochinchina and renamed into the Republic of South Vietnam next year, to be merged with the protectorates of Annam and Tonkin in 1948 under the Provisional Central Government of Vietnam, which in turn was replaced with the State of Vietnam in 1949 [1, 2]. The original flag was with three blue stripes on yellow field and was certainly in use from 1 June 1946 to 31 October 1946 [3, 4]. For the period from 31 October 1946 to 2 June 1948, the opinions differ: some sources  claim that the then flag had three blue and two white stripes on yellow field, others  claim that it was actually an erroneous description and that there was no flag change, even though there are sources presented which speak in favor of the flag's existence: a badge of the South Vietnamese Republican Guard which contains the image of a shield of arms charged as the said flag pattern, only in vertical orientation . Finally, there are also those  which even attribute the flag with blue and white stripes to whole period from 1 June 1946 to 2 June 1948. The explanations of symbolism also differ: the stripes represent either the trigram for South  or the three nations in French Indochina [3, 5], although the latter does not seem reasonable for the flag which was not used in Cambodia and Laos, nor seems to have ever been introduced for use outside Vietnam.
P. Kanniks "Alverdens Flag i Farver" i.e.[kan56] shows this flag (Republic of South Viet Nam 1946–1948) under No. 530 as "Vietminh" adding in Danish what I understand as "Flag of the anti-French nationalist organization." White stripes between the blue ones.
The same author, in 'A Handbook of Flags' (probably [kan58a] - for better identification I can add that the book carries Catalogue No. 5993/U) shows a flag with the white stripes now in yellow under the same No. and name both with additional caption '(obsolete pattern).' The accompanying text says "Flag of the anti-French nationalist organization, which has now been changed to a red one with a large five-pointed gold star in the centre."
At least the white stripes have been documented... Jan Mertens, 13 December 2004