I understand what you’ve posted; the best we could hope for (with a North Vietnamese “surrender”) was that South Vietnam would have to deal with its own communist problem instead of dealing with North Vietnamese interference as well.
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.