China supported North Vietnam in the struggle with South Vietnam. When the US pulled out in '74(?) and the reunification of Vietnam began, China would not lose one of her satellites regardless of the cost. Plus, in China, humans are expendable because there are so many of them. Human lives are nothing to the Chinese. And, Vietnam belongs to China. China will not give up the resources of Vietnam.
The North Vietnamese got MUCH more support from the Soviet Union than China. They never really were a Chinese satellite.
I'm sure any Vietnamese will beg to differ.