That's dehumanization. And according to the UN charter, countries aren't entitled to security. It is true that the threat of violence trumps everything but having past grievances live for no reason brings nothing either. I don't think China would lose anything from apologizing. It would definitely soften some Vietnamese though.
Apologize for what? For not sitting around and do nothing about Vietnam's massive ethnic cleansing campaign against ethnic Chinese?
Ethnic Chinese made up 5.3% of Vietnam's population in 1970, but only 0.78% today.
Vietnam's mass ethnic cleansing campaign against ethnic Chinese started in earnest shortly after its unification in 1975 and peaked in 1978-1979. The famous "Vietnamese boat people" refugees of the 1970s and 1980s were actually mostly composed of ethnic Chinese. By 1980, there were already 260,000 refugees in China and 400,000 in Southeast Asia. These were only the lucky survivors. The majority of the boat people, an estimated 50% to 70%, perished at sea. The boat people refugee crisis would continue throughout the 1980s.
It was not a coincidence that the Sino-Vietnamese conflict started in 1979, at the peak of Vietnam's ethnic cleansing campaign, and ended by 1990 when the flow of boat people refugees from Vietnam stopped.
China is already bending over backwards to have good relations with Vietnam by overlooking these past events. You never hear Chinese government or media talk about how Vietnam starved its ethnic Chinese population by cutting their food ration tickets (which were required to buy food) and banning them from jobs, or how the streets of Cholon, the ethnic Chinese neighborhood in Ho Chi Minh City, was once covered in corpses.
Is it a good thing for China to sweep such things under the rug for the sake of better relations with Vietnam? I don't know. But to have China actually apologize for Vietnam's atrocities against ethnic Chinese would be absolutely crazy.