I'm pretty sure in 1975 both Vietnam and Cambodia were communist states as well as Vietnam coming out from a nasty war with the US so why would Vietnam "help" the US by invading Cambodia other than for land and toppling the Khmer Rogue to try and put a stop to the genocide that was happening?
The above answer does a very good job of explaining the context of this conflict but does not do much toward contradicting some of the assumptions in your question - namely that there were Vietnamese intentions to ‘stop a genocide’, therefore it must have been a ‘humanitarian intervention’.
While this has been argued by some scholars, I find the evidence that this was the intention of the Vietnamese to be unconvincing. I would argue, as others have, that it was projected more by the PRK government after toppling the CPK as a means of legitimising the invasion.
See this answer for a more detailed outline of this argument