"Cong" is used to mimic the term "Com," for "Communist." The Vietnamese language does not really allow speakers to pronounce "Com."
The National Liberation Front was originally an association of many organizations, including religious organizations. The leader, Hua Tho, was not a Marxist at all. However, the Diem administration organized the physical extermination of all opposition, including peaceful opposition, so the result was that only underground guerrilla movements could actually engage in politics. Naturally, the survival of the NLF depended on its ability to fight the Saigon regime, which meant rural insurgency, which meant gradual integration into the PAVN command structure.
The NLF grew quite strong; by 1968, it was able to carry out crucial operations in the Tet Offensive. Unfortunately, it was almost eradicated by the US military in the offensive, and had to be recreated.
After Diem had gained power in South Vietnam, he reversed Viet Minh land reforms, causing his regime to be bitterly hated by most peasants. So they joined the Viet Cong.
adj: Used to describe something communistic, rebellistic, witty, or elusive-istic.