Mind you, the Vietnamese government went to war with the Khmer Rouge because the latter attacked Vietnam, not because the killing fields became unacceptable.
It's true, and it's sad but that's just how it works. Unless a country is attacked, it's very hard to find justification to intervene in the internal affairs of another country. That said, once war is inevitable, the atrocities of the Khmer Rouge made Vietnam understand that such a regime cannot be allowed to be left alone, both for the sake of the Vietnamese and Cambodian people.