Once again, you're not answering the question: how does NVN continue with the war under the circumstances outlined? And you're also ignoring the premise: North Vietnam is not occupied except for the trail terminus around Vinh. It's cut in half, basically, with a Marine Division, an Airborne Brigade, and maybe an elite ARVN division (the Vietnamese Marine Division is a likely candidate in this situation) sitting on the Trail's northern terminus. There's 1st Cav, 4th ID, 3rd Marine Division, and maybe elements of the 101st Airborne sitting on the Trail in Central Laos, along with a Thai division, and they've destroyed the base camps, supply dumps, and trail infrastructure, and oh, btw, they've shattered several of the NVA's best divisions that have tried to retake the Trail. Equally, the bridgehead at Vinh has also shattered a number of NVA divisions thanks to superior air power and naval gunfire. The VC and NVA in the south have been shredded (similar to Tet '68, with 50,000 KIA even by NVN sources). All of NVN's ports are mined, and every major military and transportation target in the North has been bombed to rubble, with the likely exception of the Hanoi and Haiphong urban centers. The rail lines to China have been cut-in multiple sections (bridges down, tunnels blown, rail cuts blown away. NVN is cut off from outside supply-as it should have been early on. Under those circumstances militarily, there is NO WAY that NVN can continue with the war.
The campaign is fought with the forces in-theater. No additional mobilization in the U.S. is needed. What part of that don't you get? Westmoreland said in his memoirs that it could have been done. All he needed was the OK from three people: LBJ, Rusk, and MacNamara. All three are petrified to the extreme of provoking Soviet or ChiCom intervention (a legacy of the Cuban Missile Crisis), so no-go. But, if intelligence arrives that indicates that there is no chance of either Moscow or Beijing intervening to save NVN (no reason for the Soviets, and the Chinese have their own internal problems to contend with), this operation can go ahead. The war ends sometime in the Spring of '68. And it ends on U.S. terms, not Hanoi's. Something that makes the '68 election much more interesting....Johnson might run again, the Doves are discredited, and Nixon remains an enigma.