Vietnamese Roundels

As the French left Vietnam in 1954, the country was divided to two parts. The southern part (South Vietnam) formed the "Armee de l'air Vietnamienne" and adopted the marking used since 1950 under French supervision which were variations of the national flag, i.e.  an orange disc charged with three red thin rings; the national flag was used as rudder marking.

The northern part formed the Vietnam People's Air Force and adopted a yellow star bordered red as its roundel and fin flash. See http://airwar.hihome.com/airwar/vietnam/viet15-mig21.jpg

In 1962, the south, now as the Vietnam Air Force adopted a new roundel based on the American one, white star on blue disc and orange bars bordered red and central red line. The rudder marking was the same as before ( http://airwar.hihome.com/airwar/vietnam/viet08-a37.jpg

North Vietnam changed its roundel in 1965 to yellow star on red disc bordered yellow and red bars bordered yellow with no fin flash

In 1975, the Vietnam Air Force was dissolves, and the Khong Quan Nhan Dan Viet Nam (as it has been called since 1976), kept using the former "north" marking.

Main source:
Military Aircraft Insignia of the World [cos98]
See also: http://www.fighter-jets.de/fighterjets/luftwaffen.php?land=vie&attr=vie&teil=v
Dov Gutterman, 28 June 2004

Yellow bordered red roundel with a yellow five-pointed star in the middle reaching the edges of the red disk. Behind all a horizontal red bar with yellow border.

According to Military Aircraft Insignia of the World [cos98] the pre-1965 pattern used since 1955 was a red bordered yellow five-pointed star.
Željko Heimer

South Vietnam used a 1:1 variant of the national flag as its fin flash.
Dov Gutterman, 11 February 2000