Nobody seems to have pointed out the obvious historical angle where China and Vietnam have been long-time enemies.

The issue goes back to the Cold War era and the Sino-Soviet split and it’s kinda hard to synthesize in a few short paragraphs, you can read more on the wiki article about it, but these sections could be a good summary:

Vietnam was an ideological battleground during the 1960s Sino-Soviet split. After the 1964 Gulf of Tonkin incident, Chinese Premier Deng Xiaoping secretly promised the North Vietnamese 1 billion yuan in military and economic aid if they refused all Soviet aid.

During the Vietnam War, the North Vietnamese and the Chinese had agreed to defer tackling their territorial issues until South Vietnam was defeated. Those issues included the lack of delineation of Vietnam’s territorial waters in the Gulf of Tonkin and the question of sovereignty over the Paracel and Spratly Islands in the South China Sea.

And also:

In the wake of the Vietnam War, the Cambodian–Vietnamese War caused tensions with China, which had allied itself with Democratic Kampuchea. That and Vietnam’s close ties to the Soviet Union made China consider Vietnam to be a threat to its regional sphere of influence. Tensions were heightened in the 1970s by the Vietnamese government’s oppression of the Hoa minority (Vietnamese of Chinese ethnicity) and the invasion of Khmer Rouge-held Cambodia. At the same time, Vietnam expressed its disapproval with China strengthening ties with the United States since the Nixon-Mao Summit of 1972.