China collapsing? That's a pretty big claim. And it's not necessarily a cause for celebration. You do realize that they are one of our largest trade partners. Also it varies but with Europe China is either their top trade partner or 2nd after us. If they actually collapsed, we would enter a recession just from losing trade with them, other nations all over the world would be in a depression, and then the reaction from ALL of our trade partners economies going to shit, would pull us into a depression as well. We'd probably suffer the least damage out of everyone, but it's a relative statement it would still suck massively.
As for a hypothetical war with China? The most likely I think is conflict over Taiwan, them trying to reincorporate it. On paper it seems like if it happened tomorrow we would push them back quite easily if we got involved, the Japanese, and Koreans might also send some help I don't know. But the government if it was smart would try to keep the war contained give China a bloody nose tell them they aren't top dog yet, but give them a way to save face and go back to "normal" and resume trade. If we are stupid we try to invade the mainland and regime change, at that point the Chinese may decide to launch nukes, or may decide to do what the Soviets did in ww2 and pull their government and industries away from the coast, and make any invasion extremely bloody and costly for the invaders. I don't think Japan, or South Korea would send any troops in that quagmire. South Korea would be worried looking at China's puppet North Korea, Japan probably learned it's lesson in the 30's and might laugh at America doing the same. Those are the most likely scenarios from what I know. Vietnam and the Phillipines won't really get involved at all. This is presuming a war in the near future and not decades from now while we still have technological superiority.
I have heard though from Spacebattles so I don't know if this is true, but that Taiwan is heavily infiltrated by the CCP and if there ever was an invasion half of their army wouldn't want to fight a war that would destroy their home and would just lay down their arms or even help and welcome the Chinese. I don't think it's necesarily true, but it's also not necesarily false since the Taiwanese are Chinese people they are both Han. They are racially the same like Austrians are Germans.
Other wars where China tries to invade or vassalize the South China Sea places like Vietnam, or farther in the Phillipines I think are unlikely but would be a much easier win for us since those nations are bigger, can support a guerrilla movement, and we can provide with enough weapons and support to easily help. As opposed to Taiwan which is just one island with one city which does not leave much room to maneuver in or hide, and in any serious fighting might be flattened.
I'm not an analyst however, what about you am I completely wrong and don't know the situation at all, or do I have a decent grasp, but not the complete picture?