Someone is bad at their job for thinking they can produce a world in which America's opinion counts for more in Vietnam than China's does. This is the third time I'm repeating the same thing. I don't think it's really that difficult or complex of a concept.
That's not strictly true, if you are saying that China matters more to Vietnam simply because it's in the same hemisphere.
Would it be possible for a state department to aim for a world in which Israel cares more about America's opinion than the other regional powers, such as Iran or Saudi Arabia?
Can you really not think of any other examples where a smaller country might not think, "the adversary of my adversary is my friend"?