There is some interesting parallels between Sino-Vietnamese relations and Viet-Cambodian Relations, both have to do with the bigger power forcing their culture on the weaker one and taking land.
Ask a Cambodian:
Is the Mekong Delta originally Cambodian Land?
I would suspect everyone would say yes, and they will go on for days about how it was stolen by the “filthy” Vietnamese.
Ask a Vietnamese:
Are some parts of Yunnan, Guangxi and Guangdong a part of Vietnam?
Considering the timeline for Sino-Vietnamese hostilities is much longer, I would suspect not everyone would say yes. A very clear majority would say that land was stolen by the “Filthy” Chinese.
Similar to this, if you mistake a Vietnamese person for a Chinese person there is a chance they will get mad at you and start explaining how Vietnam is different from China and all that stuff.
Mistake a Cambodian for a Vietnamese and they will also get heated, but I suspect they will get even more heated as Vietnamese people share a lot of culture with China, so the hatred might be a bit muted in some aspects (I heard some Vietnamese people talking about the reason they are so much better off is because of Confucian values and expressing some pretty racist and supremacist views at the same time). For Cambodians and Vietnamese, their cultures are incredibly different.
Cambodia is an Indic-based culture, like Thailand and Burma. Vietnam is a firmly Sinosphere culture being very culturally close to Southern China.
Historically speaking, Vietnam’s Sinic culture looked down upon Cambodia as a barbarian culture that must be assimilated.
During the highly Sinophillic Nguyen dynasty, the king Mihn Mahn (That’s the Vietnamese translation of the name: Ming Meng) hoped that:
“We must hope that their barbarian habits will be subconsciously dissipated, and that they will daily become more infected by Han [Sino-Vietnamese] customs."
That’s actually really quite similar to how the Chinese viewed the Southern tribes before their assimilation.
This feeling of hatred will continue to persist, historical issues aren’t easy to patch up without modernization and economic interdependence. Don’t worry however, do French and German people hate each other? Germany again and again plunged France into decades of brutal war over and over again, even decades after WWII they still had ill-feeling towards each other, but as France and Germany modernized, this hatred began to die off.
I suspect that will happen in terms of Cambodia-Vietnam and China-Vietnam some time in the future.