Yes. Endless supply from the north swooping into Cambodia around the DMZ and right back into South Vietnam. They moved at-will nearly anywhere they wanted in South Vietnam.
At will... aside from the strategic and interdiction bombing of North Vietnamese trails (officially) and Cambodian trails (semi-officially) and substantial (especially for the 1960s) monitoring of the routes.
You're underestimating the effort and dedication that it took to move sufficient quantities of freight by foot over mountain trails.
"Plan A", this was not. That would have been motorized transport, which was effectively interdicted.
That North Vietnamese logistics was able to smuggle sufficient tonnage to supply even a limited insurgency in the South wasn't inevitable. And probably couldn't have been duplicated by most other military forces.
