It's difficult to imagine any country being willing to join the US in fighting China besides Japan, unless China somehow decides to go on some kind of crazy rampage. Outside of that possibility, the US could maybe barely count on SK to allow the US to use its own US bases against China, but using SK's own assets against China short of WW3 sounds laughably unlikely. Similarly for Australia, which is additionally held back by economic/business ties with China. Vietnam is not really worth mentioning, and while India may possibly want to get into the fight, its only realistic avenue of attack is over land, which is certain folly for India given that China would not be facing US/Japanese Army or Marine forces which would essentially allow it to deploy the bulk of its land forces to the Indian theater. So in the end it would just be the US and Japan using their planes and ships.
A shooting war between the US and China would likely degenerate into a general war in the Western Pacific.
It is likely that everyone would be forced to pick a side.
Eg. it would likely be in ASEANs interest to remain neutral, but tacitly support China in keeping the sea lanes open in the South China Sea, against US efforts to shut down trade in the South China Seas.
Australia would make strenuous efforts to prevent a China-US conflict, but if it happens, they'll probably be dragged in and side with the USA.