No, that'd be a gross oversimplification. The Vietnamese stopped supporting Pol Pot first and foremost because he decided to invade Vietnam and massacre Vietnamese villages, not because of him genociding his own people. Vietnam thought their border disputes were finally going to be over when Pol Pot overthrew Lon Nol and they wouldn't have to deal with Cambodians trying to reclaim the Mekong delta and Saigon, but they were wrong. The Vietnamese invasion happened after Pol Pot sent his forces across the borders and started slaughtering Vietnamese villagers and capturing towns as well as ethnic Vietnamese in the border regions. Sending in soldiers to occupy your neighbor'd territory and slaughter its populace is a de facto declaration of war, wouldn't you agree?

But let's add some context.

Like you already know, from 1968 until around 1973 the Khmer Rouge were allies of the USSR, China, and North Vietnam. In 1970 the North Vietnamese invaded Cambodia to try and have supply lines & infrastructure escape US bombing, and they transformed the tiny Khmer Rouge guerrillas into a real army. The US bombed the Khmer Rouge heavily 1970-1972, devastating them and preventing a communist capture of Phnom Penh.

After the US withdrew from the region they continued to back the Lon Nol military regime while Cambodia's ousted King Sihanouk (in exile in China) was more or less friendly to the Khmer Rouge and legitimized them with his endorsements. After Vietnam violated the Paris Peace Accords and re-invaded South Vietnam, they also sent further forces and aid to the Khmer Rouge that allowed them to capture the country. At this time the Khmer Rouge was still comfortably backed by the USSR, China, and North Vietnam and were fighting US-backed enemies.

Now this is where things get complicated. From 1975-1979 the insanity of the Khmer Rouge began to show and, like I've already mentioned, Pol Pot began taking bizarre racially motivated steps against Vietnamese, massacring both Cambodian-Vietnamese and Vietnamese in cross border raids. Another reason why their relations soured is because the Khmer Rouge party conference adopted the motion to backstab Vietnamese and draw closer to China. These things caused him to be solely backed by China and become enemies of Vietnam and the USSR. China, with King Sihanouks blessing, was sending armaments and aid directly to the Khmer Rouge at this point. Compare with the Pathet Lao, which was getting weapons only via Vietnamese forces.

So in late 1978, Vietnam-Cambodian tensions boiled over and the Vietnamese invaded and overthrew Pol Pot's regime, replacing it with a far-less brutal but still authoritarian communist proxy state. At this point, they had absolutely no reason to support Pol Pot, who was outright threatening their territorial integrity like Lon Nol before him did. The Chinese invaded Vietnam in retaliation and the Khmer Rouge continued to fight the Vietnamese, aided by China and now Thailand (who previously fought Pol Pot but was now worried of Vietnamese expansionism). And you know how the rest of it went.