Just so you'll know, hardly anyone living in that era (Time magazine, Life magazine, TV, etc.) never used that term because it sounded like the "National Football League", the NFL. And football was popular in America back then. But to the very rare category of historians for the Vietnam War, you're most likely referring to the VC (Viet Cong). And I say most likely, because in this day and age of the computer, etc. National Liberation Front could be or mean almost anything...even a Civil Rights Movement or womens liberation movement from the 1960s.
But presuming you're talking about the VC, they were communists living in South Vietnam.
They were not members of the NVA (North Viet Army). The war consisted of two basic enemies: VC & NVA. The war started via the VC (always supported by the NVA), but after the Tonkin Gulf Incident in '64, the NVA were DIRECTLY involved at all times. And after the Tet offensive of '68, the NVA shouldered nearly the whole war (since the VC were nearly wiped out during their offensive).
That "...National Liberation Front..." term (title) was quite common back in the 1970s, even home grown Hippie/type groups used those headliners. Civil Rights/Women's Lib groups, etc. Shootouts in LA involving Patty Hearst was akin to those "titles."
If you're referring to the Vietnam War itself, the Viet Cong were the closest to such a group, having nothing to do with the NVA (North Vietnamese Army-a government agency like the US Army is to America) other than helping them.
they ate pigs
xdhmfgyfgh
Viet cong
There have been 22 organizations in as many countries since WW2 calling themselves "The National Liberation Front". Coming up with an original name apparently is not as easy as it looks. The mother of them all was the Viet Cong, composed of North Vietnamese and intended for the liberation and unification of Vietnam. It you are asking about one of the other 21, please specify.
National Liberation Front is the politically correct name for the Viet Cong. A group opposed by the United States
Sandinista National Liberation Front was created in 1961.
Gorkha National Liberation Front was created in 1980.
Uganda National Liberation Front was created in 1979.
Unitary National Liberation Front ended in 1945.
National Liberation Front of Tripura was created in 1989.
National Liberation Front of Angola was created in 1954.
National Liberation Front - Greece - was created in 1941.
Ogaden National Liberation Front was created in 1984.
Spanish National Liberation Front was created in 1964.
National Liberation Front is the official name for the Vietcong.
Eelam National Democratic Liberation Front was created in 1987.
United National Liberation Front - Paraguay - was created in 1960.