Because the higher echelons of Vietnam power know that the Americans come and go (see Saigon), while the Chinese are there to stay. Granted, I would have thought that the same policy had been adopted by people holding power in Kiev (when it comes to Russia), and yet, here we are.
There’s also the fact that Vietnam leaders are, for all intents and putposes, communists in name and in legitimizing ideology, I wonder how would the US sell that alliance at home (“we’ve allied with the good communists to defeat the bad communists”? who knows?)
nah. US is still backing NATO, harder than ever, and standing with Korea and Taiwan.
they didn't have much motivation to keep taking casualties in a colonial war that the French drew them into. but actual treaties matter, and the US backs those.
hell, the US tried for close to 20 years in Vietnam, starting with advisors to the French in the late 1950s. and another 20 years in the AFG and Iraq. if anything they're too willing to piss away time and money on lost causes.
There’s also the fact that Vietnam leaders are, for all intents and putposes, communists in name and in legitimizing ideology, I wonder how would the US sell that alliance at home (“we’ve allied with the good communists to defeat the bad communists”? who knows?)