the Vietcong (Việt Nam Cộng-sản "vietnamese communists") were referred to as VC or "Victor Charlie" according to the NATO phonetic alphabet. "Victor Charlie" eventually got simplified to "Charlie".

In the NATO "Phonetic" alphabet, adopted in 1956, "Victor Charlie" is the spoken designator for "VC," which stood for "Viet Cong." Victor Charlie became abbreviated to Charlie in the course of the war.

Source: "Hunting Charlie: Finding the Enemy in the Vietnam War," Pritzker Military Museum & Library,, retrieved from https://www.pritzkermilitary.org/explore/museum/past-exhibits/hunting-charlie-finding-enemy-vietnam-war/who-charlie/, Accessed 21 July 2020