Viet Cong (vēĕt` kông), officially Viet Nam Cong San [Vietnamese Communists], People's Liberation Armed Forces in South Vietnam. The term was originally applied by Diem's regime to Communist troops (about 10,000) left in hideouts in South Vietnam after the Geneva Conference of 1954, following the French Indochina War (1946–54). Most Communist troops, according to the agreements, had withdrawn to North Vietnam. Supported and later directed by North Vietnam, the Viet Cong first tried subversive tactics to overthrow the South Vietnamese regime, then resorted to open warfare (see Vietnam War Vietnam War, conflict in Southeast Asia, primarily fought in South Vietnam between government forces aided by the United States and guerrilla forces aided by North Vietnam.
..... Click the link for more information. ). They were subsequently reinforced by huge numbers of North Vietnamese troops infiltrating south, and aided in the reunification of Vietnam following the collapse of South Vietnam in 1975.

Viet Cong

in full Viet Nam Cong San
(English; “Vietnamese Communists”)

Guerrilla force that sought to reunify North and South Vietnam under communist leadership from the late 1950s through 1975. Originally a collection of various groups opposed to the government of South Vietnam's Pres. Ngo Dinh Diem, the Viet Cong became the military arm of the National Liberation Front (1960) and later of the Provisional Revolutionary Government (PRG; 1969). Members were recruited largely from South Vietnam, but they received guidance, weapons, and reinforcements from the north. The Viet Cong's guerrilla war against the South Vietnamese government and its powerful U.S. allies was successful; the U.S. withdrew its troops from Vietnam between 1969 and 1973, and the PRG assumed power in South Vietnam in 1975, following a full-scale invasion. It became part of a National United Front the following year.