When writing about the Vietnam War, it has become increasingly more common (thankfully) to use the proper terms that the organizations used themselves. The Viet Cong was never an official name, it was a nickname given to the armed wing of the National Liberation Front, the People's Liberation Army of South Vietnam (PLAF), by so-called "Free World" forces. Same thing with the North Vietnamese Army, who were officially called the People's Army of Vietnam (PAVN). Additionally, it's wise to also mention the proper contemporary name on the Vietnam the soldier fought in. For example, a scholar would call it South Vietnam (or, the Republic of Vietnam, which existed between 1955 and 1975) or North Vietnam (or, the Democratic Republic of Vietnam). I would be very happy to help you find the proper term to use. What do you have so far in terms of evidence?