During the Vietnam war, China was against Vietnam and favored its enemies.
The People's Republic of China provided resources and training to North Vietnam, and in the summer of 1962, Mao agreed to supply Hanoi with 90,000 rifles and guns free of charge. After the launch of the America's Operation Rolling Thunder in 1965, China sent anti-aircraft units and engineering battalions to North Vietnam to repair the damage caused by American bombing, rebuild roads and railroads, and perform other engineering work, freeing additional hundreds of thousands North Vietnamese Army units for combat in American supported South Vietnam. 
