The US thinks they can weaponize them against China, like they weaponized China against the USSR. And while they might have kept their relationship from getting as close as, say, China and the DPRK, they will ultimately fail at making them enemies. Honestly, I think Vietnam is just stringing the US along in exchange for relief from hostilities. But they will be targeted again sooner or later. The NED's already in there stirring up shit.