Unfortunately, the communist problem in South Vietnam is intrinsically tied to the North Vietnamese as well, since the communists in South Vietnam answer to the North Vietnamese, and by extension to the Soviets and to a lesser extent the Red Chinese. Even if they were to deal with the Communist problem in South Vietnam, more will just come in from North Vietnam.
The fact is the communists of South Vietnam (Viet Cong) itself were strong in numbers (both they and the North Vietnamese were armed by the ChiComs); the ARVN couldn’t deal with the Viet Cong by itself. The ARVN couldn’t secure the borders against infiltration.