politics

It is little comfort that China either did nothing or supported a smaller nation's communist movement if that movement (a long with Chinese material) is destroyed by USA intervention and results in a permanent USA base aimed at China

It is even less comfort in case the PRC actively helps NATO crush a communist movement and establish a permanent NATO base aimed at itself.
If you calculate that the ally will surely be defeated, then it's better to not waste your resources on helping defeat them.

Furthermore, Vietnam was hardly in immediate danger of being crushed by NATO at the point when the PRC decided to act against it.

It's an explicitly anti-imperialist instead of pro-communist approach

If you actively help the worst empire in the world, the approach is not anti-imperialist.

If the analysis is such that the USA would intervene and establish bases if a communist revolution occured in a small nation around China, the biggest threat to China is not the end of that revolution but the subsequent military occupation of their neighbor by the USA

The revolution already occurred in both Afghanistan and Vietnam, and the PRC sided with giving the control of Afghanistan to NATO's Mujahideen, so both of these cases contradict your guess.

So, if China instead occupied their neighbor, it takes up the space on the board

Did the PRC move to occupy Afghanistan or any other such neighbour where they helped defeat a local communist movement? Did they occupy Cambodia, at least after the overthrow of the Khmer Rouge by Vietnam?

I'm sorry, but this foreign policy was bad, and I can't say that you are doing a good job providing a solid explanation for why it wouldn't be so. This is one of the few issues I have with the PRC.