Once the Sino-Soviet Split got underway, China's foreign policy became more contrarian. This is part of the reason why China made closer ties with the USA than the USSR did. It's also part of why China supported the Khmer Rouge while it was in power. Kampuchea was the only socialist* country aligned with China instead of the USSR.

A newly unified, Soviet-aligned Vietnam put an end to the Khmer Rouge, and this was the foremost cause for the ensuing border conflict between Vietnam and China. There were other reasons but this was the major one.

In Vietnam, for centuries they have seen the large political entity to their north as the biggest threat. But it has been over 40 years since any military hostilities, and the only remaining territorial disputes are over small atolls far off the coast. Some reconciliation efforts have been underway, and with the Trans-Pacific Partnership scrapped and the Shanghai Cooperation Organization on the rise, we may see China and Vietnam in friendly alignment in the next few decades.