The youth's flag was used in Vietnam before 1946, led by the communist party. It looks like this.
"Nzhuynh," in an E-mail from Vietnam, 6 December 2003
The question is if this is the flag of the Vanguard Youth, or was there additional organizations? Phil Nelson, 8 December 2003
Yes, the red star in the golden pattern is the flag of Thanh Niên Tiền Phong, the Vanguard Youth.
"Nzhuynh," in an E-mail from Vietnam, 14 December 2003
The flag of the organization is described in the book Vietnam 1945: The Quest for Power by David G. Marr – "...the Vanguard Youth was permitted by the Japanese to be much more political in orientation, displaying its own flag (yellow with a red star), hanging banners across the street calling for "National Liberation" (Giai Phong Dan Toc), and publicly exhorting members to "reorganize society, improve the lives of our countrymen." Randy Young, 15 July 2016 Source:Vietnam 1945: The Quest for Power by David G. Marr, on Google Books
There were several communist youth organizations active in Vietnam before 1946. Founded by Ho Chi Minh was the Communist Youth League (Vietnamese: Thanh Niên Cộng Sản Đoàn) founded in 1926 and the Vietnamese Revolutionary Youth League (Vietnamese: Việt Nam Thanh Niên Cách Mệnh Đồng Chí Hội). In 1930, the Communist Youth League became the Annam Communist Party (Vietnamese: An Nam Cộng Sản Đảng), stating as a party within the Revolutionary Youth League.
The other major one that I am aware of is the Vanguard Youth (Vietnamese: Thanh Niên Tiền Phong) founded in 1942 in Saigon. Phil Nelson, 8 December 2003
Yes, the red star in the golden pattern is the flag of Thanh Niên Tiền Phong, the Vanguard Youth. There were 3 separated communist parties in Indochina before 1930: Đông Dương Cộng Sản Liên Đoàn (Indochina Communist League), An Nam Cộng Sản Đảng (An Nam communist party) and Đông Dương Cộng Sản Đảng (Indochina communist party). "Việt Nam Cách Mang Đồng Chí Hội" as I know, is only a campaign founded by Ho Chi Minh after he returned to Vietnam to against the French domination. After 1930,the 3 parties united and become the only party in Vietnam until now.
"Nzhuynh," in an E-mail from Vietnam, 14 December 2003
Doing a little more research, the Vanguard Youth appear to have been originally created as an effort by the Vichy French colonial government during the Second World War to insulate the Vietnamese youth from the ideas of the Japanese occupation armies across East Asia; ideas that could have threatened to undermine the colonial regime and the continuance of France's possessions there. [1] The original organization focused on sports and establishing a Vietnamese national identity with which to combat the Japanese influences. [2] By 1945, that plan had backfired on the French, creating a strong Vietnamese nationalist movement that not only successfully resisted the Japanese, but then started agitating against French colonial rule in southern Viet Nam as a communist counterpart to the northern Viet Minh. [1, 3] The Vanguard Youth eventually joined the Viet Minh following the August 1945 Revolution and effectively ceased to be an independent organization by 1946. [1, 2, 3]
Sources:
[1] The Vietnamese War: Revolution and Social Change in the Mekong Delta, 1930-1975 by David W.P. Elliott, on Google Books
[2] Wikipedia article on the Vanguard Youth (Vietnam) in English – https://en.wikipedia.org/wiki/Vanguard_Youth_(Vietnam)
[3] Vietnam 1946: How the War Began by Stein Tonnesson, on Google Books Randy Young, 15 July 2016