The Soviets, fearing their credibility with satellites and client states is being challenged, back the Vietnamese militarily. If it escalates and goes nuclear. After that, all bets are off as any large scale launch against the Chinese by the Soviets might be accompanied by a launch against the US if they fear the US will take advantage of Soviet weakness and launch against them after a Chinese strike.
The biggest issue is Brezhnev's temperament is too conservative. I think you need someone more radical and willing to take a risk in the situation - go big or go home mentality. So I think your POD has to be a different Soviet leader. No idea who among the Soviet elite in the 1970s could have been elevated to GenSec and had that personality. Brezhnev got rid of a lot of them in the late 60s and early 70s and frankly, most of the Soviet leadership by then just wanted to live out their days as they were all pretty old by then. But as WWIII scenarios go, its not the worst one ever proposed.