Assessment of the 1988 scenario. The PLAN curb-stomps the locals. They have the numbers and their shore-based PLAAF air is better than anyone (except the Americans who tangled with it on several occasions.) assumes.
Chances of the Americans coming in to redress the result? 40-60 against. The Americans think they still need the PRC and there is no upside.
The US is not helping Vietnam in 1988, the US did not even have diplomatic relations with Vietnam and a halfway decent chunk of the American public who had been raised on a steady diet of Chuck Norris and Sylvester Stallone movies believed the Vietnamese were still holding American POWs. Nobody batted an eyelash over the dustup that actually occurred in 1988, if the Chinese grab a few more Vietnamese held islands, nobody else is going to care.