That was Johnson's fear, certainly. Whether it was actually realistic- China was in the middle of the Cultural Revolution. They were busy senselessly slaughtering each other in as purge of unbelievable extent and stupidity.
The U.S. Army was in a strange place around Vietnam- doctrinally this is kind of the end of the tactical nuclear age, also the end of Massive Retaliation, insanely vicious when the U.S. had no- one against it and insanely dangerous once they did;
also when they are coming to the realization that the cold war will include a lot of limited actions, proxy wars, even, well, the much maligned term "police action" starts to make sense when actual capital-W War is going to be SAC/NORAD versus RVSN/Voyska PVO.
The U.S. Army, I have seen it suggested (admittedly by a USAF partisan), leapt at Vietnam as a heaven- sent excuse to rebuild their force structures, regain lost ground in interservice politics and lost budget, and deliberately adopted a strategy that would require huge quantities of soldiers, the larger the better. Numbers were a feature, not a bug.
At this point you have to recall that the U.S. lost the Vietnam War by winning all the battles except the invisible ones. In accounts of the war, cockups are legion, but actual defeats very few and far between.
What they couldn't do, and given the domestic imperatives may not even have really tried to, was make the south Vietnamese government look like anything other than an American puppet, or hurt the North badly enough to change it's political imperatives.
The war was not an extension of Vietnamese politics, dubious as Cold War politics, or even American politics in any sense outside the walls of the Pentagon. Even the victories made no sense.
Could they have staged an invasion? Piece of piss. Easy. With the fire support the Navy and Air Force could bring to bear, the NVA would have been thrown into the fight in their hundreds of thousands and slaughtered in their hundreds of thousands just as fast.
North Vietnam would have had a lost generation and then some; the demographics of the place would have been permanently altered. (It was bad enough as it was.) And that might have been the only real effect on the politics, because what do they do then? There's no evidence they had much of a clue for an end game.
Winning the war would have been the easy bit. Westmoreland et al didn't even understand the problem well enough to begin to realize they needed a better plan for winning the peace.
