Vietnam has almost always hated China. Vietnam only suffered under american imperialism for a couple decades, its suffered under chinese imperialism for millennia. There was a sort of alliance of convenience during the Vietnam War but as soon as the Americans left they sided with the soviets when the sino-soviet split happened. This got worse as China ( and the US to a lesser extent) backed Polpot and the khemir rouge while they were genociding Vietnamese in Cambodia and repeatedly starting border skirmishes until the Vietnamese invaded and kicked out polpot.

Even today the Vietnamese have contested borders with China in the south China sea, and the see the US as the only country that can guarantee they're claims.