Bro without a fear of Chinese intervention, the U.S would have launched a massive ground invasion. This is not something im just pulling out of thin air, i have read alot about the Vietnam war. The truth is if the communist didnt prevail over the Chinese nationalists(KMT) during the Chinese civil war, then the KMT government would have been ruling China and be closer to the U.S . In this respect, the north Vietnamese communist would never have grown stronger in the first place(since they wouldn't have had any major backer/support) and would have been limited to be a small insignificant militia. which would have never posed a real challenge to the south Vietnamese government/troops, much less the U.S. You seem to underestimate the support the communist North vietnamese got from both China and Russia. The border with China was wide open through out the war and there was a free flow of weapons/personnel/equipments etc from China to north Vietnamese fighter(Vietcong) throughout the war, and the U.S was limited in its actions over the north, since it couldn't bomb/block the flow of these arms/equipments from the Chinese border fearing a Chinese reaction/intervention like in Korea. So this limited U.S actions in the north alot, without which the U.S and South Vietnamese forces would have easilly wiped out the north Vietnamese forces.
The fact that a huge country like Chinna borders vietnam and was openly supporting/arming/financing the north Vietnamese(together with Russia) in itself should have made the U.S know it would never have come out of this war victorous. The war not only destroyed vietnam but set it back decades behind(followed by sanctions and isolation), countries like South Korea and Japan benefitted enourmously from this war by supplying/producing lots of equipments for the U.S.
Just ask yourself a simple question: Had China not truned communist (i.e had the KMT government won the war) do you think Vietnam would have been communist today? let's be honest. The answer is NO. The vietcong would have been defeated long ago by the U.S and south Vietnamese soldiers, and Vietnam today would have been a proseprous democracy just like South Korea, Taiwan and other Asian tigers. Vietnam missed that boat unfortunately, due to mainly to China.
The truth is had China not turned Communist in the first place, then im sure Vietnam,laos , cambodia, North Korea and other Asian countries would have never been communist. Mao's victory over the nationalist, really set back several Asian countries, since this led to the spread of communism and communist guerillas(Maoist) in several asian countries(phillipines, Indonesia, India etc). Had the KMT won, Asia would have looked very different today and more prosperous indeed. Unfortunately for Vietnam it wasnt to be the case.