Vietnam War, started in 1959 and extended until 1975, was the confrontation between North Vietnam and South Vietnam, both fighting for the unification of the country under their control. The conflict in Vietnam counted, from 1965, with the active participation of the United States army, which fought alongside the troops of South Vietnam against the forces of North Vietnam. American action in this war was part of its policy of containing communism at the international level during the Cold War period.
During the early years of the conflict, the United States was not directly involved and maintained its role only in the supply of weapons and in the action of American advisers who prepared South Vietnamese troops for battles. Despite American support, the Viet Cong troops (name used in reference to the North Vietnamese Communists) had the upper hand in the conflict.
I believe the China this question is referring to is The Peoples Republic of China. The peoples Republic of China had supported North Vietnam despite the Soviet-Sino split taking place years before.
