Vietnam went to war against the Khmer Rouge. They deposed Pol Pot. Vietnam also went to war against China. So much for The Domino Theory. Continued instability is a poor justification for continued occupation.
Uh the viet cong(as did jane fonda and other American leftists) initially supported pol pot. It was the American left who were joyous that pol pot took over Cambodia, but you can go ahead and try to revise history, to the detriment to the 1.5 millions innocent Cambodians slaughtered by the khmer rouge.