Air Vietnam was founded in 6 May 1951 and worked until 30 April 1975. This airline was the national carrier for South Vietnam.
Until the mid-1960s, the green and white flag was used. Then the green-yellow flag was used. This flag had with the red waves on yellow background a patriotic national reference to South Vietnam. The inscription on the aircraft was changed from "Air Vietnam" to "Hàng Không Việt Nam." In 1968, the French company "Air France" acquired some business shares of Air Vietnam. The layout of Air Vietnam was changed on the basis of the national flag of the Republic of Vietnam. This airline flag is the modified national flag, their three strips were arranged like a bird.
In 1990, a new airline "Air Vietnam" was established from society "Vietnam Civil Aviation." In 1993, due to complaints in patent law, this new airline "Air Vietnam" was renamed in "Vietnam Airlines." Jens Pattke, 18 January 2015
Apparently there was an earlier (1950s) version of the green-and-white flag, which was similar in layout to the Air France flag, but with the dragon badge at the centre and the letters "VNHK" in the four quarters. Attached is a photo close-up of the apparent 1950s Air Viet Nam flag alongside the Air France flag. Miles Li, 20 January 2015
image located by Miles Li, 20 January 2015
image by Randy Young, 15 April 2016
Speculative graphic of the VNHK flag based on the Air France flag of the same period. Randy Young, 15 April 2016