Were they simply informed and passionate about Marxism-lenninism, were they conscripted, or did they have more personal grievances with South Vietnam and the occupation?
A lot of it was self determination. They wanted a Vietnamese led Vietnam with no puppets installed by other countries. They viewed communism as part of this self determinism, but don't discount nationalism as a motivator. Ho chi minh was actually inspired by US founders.
Okay, in answering your question, we need to explicitly differentiate between the Viet Minh and the Viet Cong. The Viet Minh spawned in during the French control of Vietnam and was revitalized with the arrival of Ho Chih Minh. They eventually transitioned into solidifying as the Democratic Republic of Vietnam. The Viet Cong was a resistance group that formed WITHIN South Vietnam out of South Vietnamese volition, with assistance of senior Viet Minh operatives and other DRV influences.
What drove Viet Cong combatants? Many were South Vietnamese natives who found the history of overall poor leadership in Saigon (Bao Dai, Ngo Dinh Diem, etc) unacceptable and were influenced by Viet Minh operatives to participate in a revolution. Other soldiers may have been ARVN (South Vietnamese Army) deserters who felt neglected and abused by low morale, harsh conscription policies and spotty leadership.
Part of Ngo Dinh Diem's attempt to make a more solid and unified South Vietnam involved extension of army conscription lengths from one year to two years and to greatly reduce time off. This is important because the demographic of most conscriptees were young able-bodied men, often from an agriculture background. This often meant sons would have to abandon their familial duties (harvest season, planting season etc.) to remain within the army. Broadly speaking; familial duty is highly prized in Asian cultures. The Family is often the most highly regarded institution and to abandon them in time of need is a matter of shame. Many documented cases of deserters were found to have returned home to more actively help the household. This blindness from the government of South Vietnam to the needs of its demographics was what led many to believe the best path for Vietnam was to support the Communist movement in the North.
I've commented about this topic before, but with a greater focus on how it made Ngo Dinh Diem a poor leader, but the same anecdote sheds a whole new light when you consider its impact on its constituents. Read Robert K. Brigham's ARVN: Life and Death in the South Vietnamese Army