China and Vietnam are ancient enemies. The Vietnamese don't like being bullied by their Goliath neighbor. The Chinese don't like it when a country they consider a satellite has its own foreign policy and that includes making friends with us.
Sure, china supported North Vietnam during that war, but that was a poke at us. Now that Vietnam is united, somewhat powerful, and rapidly industrializing China isn't so friendly any more. In fact, they have been downright unneighborly. Going so far as to start a war with them a while back.
So now, China wants all the natural resources located off of Vietnam's coast. I think the Vietnamese will fight for that. I think they'll build an Indo China alliance to stop Chinese aggression. We'll see what the future brings.