I think the answer I wrote about Vietnam's invasion here kind of answers this one as well. So feel free to check that out, just as a quick addition/summary however, Vietnam was aware of the situation in Democratic Kampuchea. To an extent even those in the West were aware after refugee reports continually spoke of the same kinds of horrors. But the Vietnamese invasion was also built upon a significant amount of those Khmer Rouge who had defected eastward. The reason I said 'situation' as opposed to knowing about the 'atrocities' generally, is I guess just to cover the fairly wide range of abuses that were committed in DK, some however were secret acts of the state, some were wide ranging policies that any refugee or person privy to the function of the regime would have been aware of.

For example, the discovery of S21 in Phnom Penh was certainly not something they would have been aware of prior, and while there were hundreds of security offices around the nation, the apex of these was S21 and it was top secret. The Vietnamese photographer who stumbled upon the grounds of the former high school did not know what he was going to find.

Contrast those 'atrocities' to, say, the forced labour nation wide or widespread starvation in many areas by 1978 and you have clear examples where the Vietnamese and their Khmer counterparts would have known what was happening prior to the invasion.