China collapses following the cultural revolution. Then what?

So, how does the world react to a Chinese collaps. Does the UN intervene? Does the USSR set up a new government or tries to stay out and let things sort themselves out on their own?
 
If China were to collapse, then the odds of a US invasion of NOrth Vietnam go up. THe primary reason the US didn't do so in OTL was because we were afraid of a Soviet invasion of Vietnam.
 
Outside the wild fantasies of internet edgelords and hardcore sinophobes, China was never going to "collapse" during the Cultural Revolution.
 
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