From Wikipedia

Vietnam's Bloody Secret

Vietnam Era veteran is a phrase used to describe someone who served in the armed forces of participating countries during the Vietnam War. The term has been used to describe veterans who were in the armed forces of South Vietnam, the United States armed forces, and countries allied to them, whether or not they were actually stationed in Vietnam during their service. However, the more common usage distinguishes between those who served "in country" and those who did not actually serve in Vietnam by referring to the "in country" veterans as "Vietnam veterans" and the others as "Vietnam era veterans". The U.S. government officially refers to all as "Vietnam era veterans".

In the English-speaking world, the term "Vietnam veteran" is not usually used in relation to members of the communist People's Army of Vietnam or the National Liberation Front.