Its kinda sad but geo politics is funny the Vietnamese really do hate the Chinese for what they did in 1979 regarding the border tensions and Beijing support of the Khmer Rouge and Pol Pot
Before 1975, North Vietnam was an ally with China and the Soviet Union
After 1975, Vietnam was an ally with the Soviet Union and against China
Reason:
Before 1975, North Vietnam needed aid from two nations, to fight South Vietnam and the United States
After April 30, 1975, the north and the south united.
At the time, the Soviet Union was a superpower, China was the third world nation, and the two countries had many conflicts. If Vietnam shows a clear attitude, sided with the Soviet Union, Vietnam will receive more benefits.
Another reason is the South China Sea (Vietnam calls it "EAST SEA/Biển Đông"). China claims all of the South China Sea. Before 1975, North Vietnam did not object because North Vietnam needed aid from China, but after 1975 it was different. Many small islands in the South China Sea are occupied by South Vietnam. North Vietnam defeated South Vietnam, they felt that the winner should receive all the assets from the loser. The South China Sea has many resources, no one rejects this benefit.
Therefore, Vietnam claims sovereignty over the South China Sea(Biển Đông) and is ready to conflict with China. Lê Duẩn believes that with the support of the Soviet Union, Vietnam will win. Seizing property and deporting people of Chinese origin (người gốc Hoa/Hoa Kiều) are the first thing he does.
Eventually, the two nations from allies, became enemies.
Many people believe that: China and Vietnam became enemies starting in 1979 after the Cambodian-Vietnamese War (Chiến tranh biên giới Tây Nam) and Vietnam-Sino war (Chiến tranh biên giới phía Bắc). This is a mistake. The problem started in 1975 and may be before 1975