Goodness gracious, we are suspicious of the Vietnamese. The reason the government of Vietnam invaded Cambodia on Dec. 25, 1978, was because the Khmer Rouge had been crossing the border, attacking Vietnamese villages, and killing Vietnamese citizens. Almost any government would respond militarily to this.
And even though stopping genocide was not the reason, Vietnam’s invasion of Cambodia still probably stands within the top five best examples of straight-up military action to stop genocide in its tracks, let’s say within the last hundred years. The top example is the Allies defeating Nazi Germany in WW2.
LATER EDIT: Yes, it was highly fortunate that Vietnam invaded and defeated the Khmer Rouge in most, but not all, of Cambodia within just a few short weeks. However . . . Vietnam then misplayed their hand by continuing to occupy the country all through the 1980s. They should have made a big deal out of asking for UN peacekeepers, and taken abundant photos of starving Cambodians.