USSR uses nukes, most likely bombers, maybe IRBMS, does a limited decapitation/counterforce strike. Notifies the USA one hour before bombs drop/missiles impact. China tries to retaliate with the few bombs they have, Soviet air defense shoots all bombers down. PVO Strany gives it 110%. China still doesn't sue for peace. Soviets take out a city a day until the Chinese give in. Takes maybe a week. China now Soviet satellite state.
USA really can't do anything, being mired in Vietnam. Fallout could be a problem if there were ground bursts, affecting Soviet Far East, Korea, Japan, western/northwestern USA, maybe Taiwan/Philippines.
USSR - they broke it, they bought it. IMHO "their" China will be one hell of a mess. USSR may have to stop supporting North Vietnam. If that happens NVN eventually agrees to a peace of some sort. At best, they pull totally out of the South. USA/Allies and ARVN can now finish cleaning up on the Viet Cong, who got wrecked during Tet in 1968. Once Viet Cong have been reduced to manageable size we continue pullout of ground forces.