Being a neighbor of a global hegemon with common cultural roots doesn't guarantee annexation/assimilation. There's Ireland and the UK. And there's Canada and the US. These countries survive thrive as independent nations even when they recognize the influence of their neighbor. And, both take pride in skewering the hegemon when it suits them.
So, why can't Vietnam play the same role here? It might, like Canada during the OTL Vietnam War, allow Chinese draft dodgers fleeing conscription into an unpopular imperialist war. Or, like Canada's relations with Cuba, openly trade with whatever country China declares an enemy.
Heh. Actually I think the better example for Vietnam would be Cuba itself. Just to scale, if nothing else. Like the rest of Central America and the Caribbean, the choices were toeing the US line or having the economy and foreign policy almost defined by the stand-off with America.