That's similar to a theory I read at another forum. After the Vietnamese signed a mutual defense treaty with the Soviets, The Chinese wanted to teach the Vietnamese that the Soviets could not be trusted.
They sent their first-rate troops to the Sino-Soviet borders, then sent in the second-rate troops against the Vietnamese. After the Soviets did not come to the Vietnamese' defense, the Chinese withdrew their troops, having shown that the Soviets will not come to their defense.
It's an interesting theory. Don't know if it is true or not.
More than theory. Soviets lost upwards of 3K KIA along border with China during Sino-Soviet War. People oft times say the Chinese wouldn't risk the lives of 25K KIA in the Sino-Vietnamese War, merely to "test" the Soviet Union's resolve (to defend Vietnam).
In fact, Mao had once said words to the effect, "What's 2 or 3 million dead Chinese when there are 800 million of us?" China reacted to what they felt was an "outflanking" by the Soviets.
This failure to act, on the part of the Soviets, caused a rift in relations with VN that has not healed to this day. It confirmed, in fact, that USSR was a "Paper Polar Bear."