That's actually what the Chinese Incursion was about.

The situation was so bad, that Communist Party, Khmer Rouge, loyalists crawled out of the killing fields, fled to Vietnam, and begged for the NVA to intervene because it really did look like the Khmer Rouge were going to kill literally everyone in Cambodia. China was fully aware of this for years and did less than nothing to stem the violence. When the Khmer Rouge began mass slaughtering the Chinese in Cambodia, China didn't even consider to lift a fucking finger.

From Thomas Sowell's "Migrations and Cultures" page 212-213

As French Indochina disappeared, Vietnam Cambodia, and Laos when their separate ways as independent states. All discriminated against the Chinese with laws forbidding them to engage in many occupations. When the Communists established control in these states during the 1970', the Chinese were doubly hated as "capitalists" and doubly targeted for oppression. Of the one million refugees who fled Vietnam between 1975 and 1979, an estimated 70 percent were Chinese. Many were "boat people" who often put themselves and their families on flimsy and leaking river craft and took the desperate gamble of setting sail on the high seas, where hundreds of thousands drowned. Many "boat people" were also victimized on the high seas by crews of other vessels who boarded their defenseless boats to rob, rape, and murder the hated Chinese.

After 1979, the exodus declined but did not stop. With the Communist takeover of Cambodia, rename Kampuchea, a new wave of mass barbarities fostered a new wave of refugees, sometimes called "land people" who tried to escape across the land to Thailand. Many were killed on the way. Those who remained were caught in the greatest mass murders of the postwar era by Communist leader Pol Pot's Khmer Rouge troops. Half of Kampuchea's Chinese population of 400,000 were killed during the 1975-1979 period. They were only part of the millions slaughtered in the infamous "killing fields" of Kampuchea. China, which protested vehemently about Indonesia's mistreatment of it's Chinese minority, made no such protests over what was happening in Kampuchea. Pol Pot was an ally.

The Vietnamese decided that the killing needed to stop because it was beyond the pale even for them, and it was clear that such instability could spread to the utterly war-ravaged Laos and Vietnam. Also, it gave them a good excuse to project power on the former Indochina region, so they invaded and overthrew the Khmer Rouge. This outraged China, (especially since Mao had taken massive steps to fund, and supply the Viet Minh's (Vietnamese Communist Party) war against America and South Vietnam) and it prompted an invasion by China, who commenced with shelling the city of Hanoi and occupied the northernmost parts of Vietnam.