Seeing claims of Vietnamese imperialism in Kampuchea is honestly very sickening. It's quite similar to slander against the USSR as a tool to minimize the horror of the Nazis, but instead it's a tool to minimize the horror of the Khmer Rouge.
The most oft-cited anti-communist mythology is the Black Book. Coincidentally the origin of much of the claims against Vietnam is the Black Paper, written by the Khmer Rouge. Much of the Black Book is from Nazi sources.
Vietnam is called the aggressor, but the Khmer Rouge claimed that S. Vietnam was actually part of Kampuchea (Kampuchea Krom), & wanted to reclaim it. That would have meant the genocide of millions of Vietnamese. 
In one Khmer Rouge radio broadcast they endorse killing every single Vienamese person. This was based on the absurd claims that in border disputes for every Kampuchean soldier killed, they killed 25 Vietnamese soldiers.
After the revolution where Kampuchean socialists, assisted by Vietnamese comrades, defeated the Khmer Rouge, Vietnam set to work helping their Kampuchean comrades in every way they could, sending supplies, machinery, technicians, etc..
The Khmer Rouge had not only destroyed much of what little industry Kampuchea possessed before the genocide, but also had killed many of the people able to operate industry. Vietnam sent people to fill that vacuum. Even liberal economists recognize the turnaround as a miracle.
I might make a thread eventually going through the Black Paper piece by piece but I'm tired. Khmer Rouge apologists make me tired. And lets be clear that's what these folks are, as without this 'Vietnamese imperialism' the Khmer Rouge would have maintained power for far longer.
