Why? You think they'd really be an ally? I don't see it. There's too much history between Vietnam and China, like North Korea and China. All communist countries.
You know, after we left Vietnam... They went into Cambodia and took out pol pot. While most of their regular troops were down south, China invaded Vietnam in the north. The Vietnamese reserves were called up. They were experienced combat veterans from the last decade of war. They beat the crap out of China. This was 1979 I think. There is no love of China in Vietnam.
Most of the history of Việt Nam has been that of fighting free of Chinese domination and kicking and keeping China out.
From BC through the tenth century China conquered and reconquered Việt Nam but Việt Nam was too remote from the Capital and the satraps and occupying armies sent south tended to become Vietnamese and to fight for independence which was a number of times accomplished for short periods. The Vietnamese finally fought free and was reinvaded unsuccessfully several times.