Vietnam invaded Cambodia in December 1978 primarily to stop relentless border incursions, attacks on villages, and massacres of Vietnamese civilians by the Khmer Rouge regime. This led to the end of Khmer Rouge, which was a VERY GOOD THING. China was HELPING the Khmer Rouge, so yes, it actually was rather "imperialist aggressiony". I haven't encountered a Khmer Rouge apologist in a long time.
The thing about Russia is they lie ALL THE TIME. The Khmer Rouge was actively conducting cross-border raids into Vietnam killing civilians. Ukraine was not attacking Russian territory. The Khmer Rouge killed 1.5-2 million Cambodians (roughly 25% of the population) in a well-documented genocide. Vietnam ended a genocide and withdrew most forces within a decade. Russia annexed territory and continues occupation.
The situations are not comparable at all.
Interestingly the same excuse Russia used for invading Donetsk and Lunhansk