China invaded Vietnam because Vietnam stopped a mad man from killing Cambodians any further (China backed Pol Pot). It was highly unlikely that Vietnam was going to exert any influence on anyone because for the most part, they just wanted to be left alone even if they were pro USSR

Nowadays many Vietnamese just dislike China because of the water conflict, although the two countries have expressed that they want to settle it diplomatically and without the interference of outside nations. China also does a lot of business in Vietnam and generally when outsiders have a lot of economic pull in your country, people tend to become upset