Support for anti communist Diem and an attempt to reunify Vietnam was taken by a South Vietnamese government that was behind in military preparedness, and frankly thinking. The Viet Cong conducted an insurgency and the South Vietnamese forces that were not under the purview of western military advisors were not up to snuff. They could not handle the aggressive and audacious Viet Cong. Their commanders thought of insurgencies in terms of the insurgencies that were conducted in World War II. Those, whether fought against the Japanese or against the Germans were guerilla operations in support of larger military efforts. They were thinking that insurgents basically protect tanks and planes and airstrips. These communists were fighting from Mao’s Red Book. There were far fewer planes and tanks and warships. The Communist Insurgency was the bigger war effort itself.
The Viet Cong insurgents successfully tapped into the gratitude and support of local peasants. They understood that access to disguises and ready access to egress from combat into civilian populations was vital to the success of the anti invasion forces. Their doctrine taught them that it took ten soldiers to kill one guerilla was a testament to their ability to garner local support, not their military prowess.
The insurgents hiding in the population achieved victory by their ability to conduct combat and combat support operations under the nose of the Americans without detection. They passed information along various routes, sometimes by just word of mouth. They could evacuate injured soldiers, provide aid, and literally be farmers by day and fighter pilots by night.