I see a couple problems with your question that make it difficult to answer. The following outlines some of them:
The NVA had fought conventionally before 75, against the French, ARVN,and the US, but reverted to a guerilla campaign and supporting an insurgency (the Viet Cong) whenever they felt outmatched or their conventional forces were seriously degraded as a result of losing battles and/or bombing. The North had to rebuild their Army several times over in the course of a decade or so.
The insurgency in the South effectively ceased to exist after Tet '68, which saw the elimination of most of the Viet Cong organization. From that point the "insurgency" was primarily a guerilla campaign waged by the NVA and not an actual insurgency.
The NVA launched several failed large scale conventional attacks against the South before, during, and after American intervention, all failed, with the last one to fail being in 1972. Nearly all were stopped by overwhelming American airpower in support of ground troops, especially in 65, 68, and 72.
"Vietnamization" of the war and support to the ARVN between 1970-1975, during which the majority of US combat troops left, actually had some measurable successes, especially when supported by American airpower, despite the failure to achieve political stability in the South. 72 is the biggest and best example, but not the only one.
The final NVA invasion of the South in 1975 was not the cakewalk they expected, in the beginning. The ARVN actually put up a pretty good fight for a while, until the US Congress passed a law forbidding President Ford from expending money to support them. That was when ARVN resistance began to collapse - when it became clear the air power that had been crucial in crushing previous conventional attacks was not coming. A similar situation helped bring about the collapse of the French (though they put themselves in a bad situation to begin with). They appealed to Eisenhower for US air support and got a firm "no" in reply.
There is no telling what would have happened had the RVN gotten US air support in 75, but it would have gone down differently, even if the ultimate result might have been the same owing to the RVN's chronic political problems.
Incidently, and as an aside, there are a lot of interesting comparisons between what happened in Vietnam in the end and with the American withdrawal from Iraq after 2008 and the rise and success of ISIS, though the mistakes of '75 were not repeated in that case, so maybe someone learned a lesson somewhere.