Soviet Union didn't want to escalate the war so they did not intervene. Vietnam was on the verge of collapse as china already secured Langson, the gate of Hanoi. soviets won't help them even if it china occupied all of vietnam.
Soviets didn't intervene because there was no need to intervene. Vietnam handled the war fine by itself, considering most of the regular Vietnamese Army units were stationed in Cambodia at the time, and the PLA mostly faced local militia.
As for the Soviet-Viet alliance, the largest overseas Soviet military base during the Cold War was at Camh Ranh Bay in Vietnam (I personally know a guy who was stationed there as part of a Tu-16 crew). The Soviet presence there was very important in limiting Chinese influence in Southeast Asia.