@ephone
You were brainwashed and propagated by CCP too much.
Let me unfreeze your ignorance.
Before 1945, Chinese were looked after by the Japanese. Chinese were not even allowed to enter park in China, how they could have any "interests" in Vietnam?
In 1946, the consent of the Allies, Chiang sent 200,000 chinese troops into North Vietnam to "disarm the Japanese military". Chiang thought that it was a golden chance to kick out French, creating influence back in Vietnam.
Note that, indeed, Japan was defeated by U.S and Soviet in China, Korea and Japan. In Vietnam, Japanese surrendered Ho Chi Minh's Viet Minh forces in 1945.
However, the agreement were signed by Ho Chi Minh and French in 1946, forced Chiang had to withdraw troops return home, soon after.
In 1949, Mao defeated Chiang, took power in China, and began to support Viet Minh resistance against France.
In 1954, the Viet Minh defeated knock out French at Dien Bien Phu. The French were forced to sit at the negotiating table. They had nothing to bargain? French hinted that, if Geneva convention is failed, Americans will intervene in Vietnam. Fears Uncle Sam intervene in Vietnam, the Chinese Communists underground handed with French to divide Vietnam. CCP calculated that it would help create a buffer zone for their safety in the southern border, despite the consequences to Vietnam War 1954 - 1975 for VCP.
On the Vietnam side, both Vietnamese Communists and Vietnamese Nationalists did not want Vietnam to be divided. But with weak force and pressure from all sides, so they had to accept the country was divided...
So, as mr. gambit said, we can say that China was a co-starter of the Vietnam War.
BTW: What are islands "China even let vn use our own islands to fight U.S"?