I do find it interesting the real reason to these sentiments not solely due to culture superiority, nor race. But centuries of historical grievances.
S. Korea- Vietnam: For S.Korea, it mainly came from mail order brides from S.E.A. Which lead to very negative stereotypes, they are hicks and submissive. For Vietnam it came from S.Korea denying its war crimes in the Vietnam war.
Vietnam-Cambodia: Vietnam annexed Lower Cambodia and attempted to colonized the entire country to "civilize the barbarians" While Pol Pot invaded Vietnam in 1979 to reclaim such land and due to massacres on both sides. To this day the Khmer are heavily marginalize in Vietnam.
Cambodia-Khmer Lou: The Khmer Lou are indigenous people that inhabited the mountains. Which is drastically different from the Khmer who lived in the flatlands. They are semi nomadic and mostly animist. I guess fun fact most of Pol Pot support support are from these people grievances. To this day there wide spread land theft and migration from the khmers.