They know where their bread is buttered both from the standpoint of capital investment/foreign trade, and offsetting China. IIRC they’ve had a serious border skirmish with China more recently than their war with us. I think it was soon enough after we left that they bloodied China’s nose, probably a result of having some of the best light infantry in the world at that instant in time.
Vietnam also did the world a favor and took out Pol Pot and the Khmer Rouge, which also angered the ChiComs.