If there was a skirmish, Vietnam is just south of China, is communist just like China, and would likely find themselves quickly aligned with China over the US. So much so they may as well be the same level of risk.
Right now China is claiming control of some of Vietnam's coastal waters including the sinking of Vietnamese fishing vessels, so there are ongoing disputes between the two countries.
Previously Vietnam partnered with the USSR against China. Now that Putin is Xi's closest ally (or wants to be seen that way) Vietnam could well be looking for new allies.
https://en.wikipedia.org/wiki/China%E2%80%93Vietnam_relation...