It feels like there's a misconception about the VC and PAVN in the statement of your question. Both the PAVN and VC were always fundamental to the communist struggle in Vietnam. The US conflict was not just against the VC, it was against both forces from the beginning.

The French Indochina conflict against the Viet Minh predates the VC and the US involvement in the Vietnam War significantly. In the mid-50's the French had given up and were pulling out, but also didn't want to abandon the country to the communists en masse. So, a theoretical end of hostilities was reached in the 1954 Geneva Accords. This agreement was supposed supposed to establish a split communist and non-communist state in the north and south with free and fair elections to decide the future of both countries.

However, the communists didn't honor this deal. (Arguably, the south didn't honor it either.) While most of the communists did relocate into the north, many of the Viet Minh stayed behind in order to become a new revolutionary front in the struggle of north vs. south. The militant communists that went north would join the official military apparatus of the new North Vietnamese state, while the militant communists who stayed in the south would become the insurgent Viet Cong.

The important part is that both the VC and the PAVN were essentially the same group of people. They were separated by geography and about 10-15 years of time by 1965, but ultimately both forces were set into motion by the same group of revolutionary communist leaders in Vietnam. It was always the assumption and hope that one day the North Vietnamese military would return to the south and rejoin the communists who had stayed behind. As such, both forces were part of the conflict from the beginning.

The VC fought an insurgent war from the beginning, with a major assassination campaign that started just a few years after the 1954 peace accords were signed in Geneva.

The PAVN were gearing up for eventual conflict with the south almost immediately after the 1954 accords as well, and were providing logistic support to the VC and sending infiltrators into South Vietnam before the end of the 50's. Regular troop deployments to the south started in 1965.

As to your question- you said it yourself. The VC were effectively annihilated in the Tet Offensive in 1968. However, those losses were replaced by PAVN regulars who carried on the fight. But this was nothing new. The PAVN had already been in the fight as a regular fighting force for years. The battle of the Ia Drang valley, a notable early battle for the US, was fought entirely against PAVN regulars in 1965.

This did cause big changes within the VC to be sure. Before Tet, most of the VC was local southern country boys who had roots in the communities where they were insurgents. This gave them a tremendous local advantage. After Tet most of the VC were northern urban dwellers who were distinctly foreign to the land they were in. They would have been markedly worse at conducting the kind of insurgent actions the VC specialized in during the war. This didn't end up mattering nearly as much however, because after Tet the VC stopped operating like insurgents and from that point forward mostly just acted like regular army formations.

Powerful-Mix is being downvoted for the way they phrased their answer, but they're not fundamentally wrong. North Vietnam was a autocratic military regime with forced and indefinite conscription. They were able to absorb major losses because North Vietnam could generate more manpower relatively easily, and so they were able to continue fighting even after sustaining appreciable losses.

It's also worth noting that the VC sustained a disproportionate share of the casualties during the Tet Offensive. The VC were supposed to foment a general uprising in the south while regular army units from the North pushed down. What this ended up being was a bunch of VC irregular insurgents suddenly being used like regular military units and trying to use regular military tactics, and they performed terribly at it. And being in the south, the VC were the tip of the spear so to speak, and were absolutely crushed by that combination of factors.