Is that confirmed though that it would collapse those economies? China itself would also incur considerable damage as trade flows both ways (i.e. loss of export value). Trade with these countries accounts for 40-45% of total Chinese trade.
But do you want to leave this to the last minute? At that stage, US vassals are ready to die by creating conflict against China, but US enemies.. are they damaging US in equal magnitude or even willing to create conflict at all? Apart from Cuba, there isn't anyone near US to create conflict either.
It's not about if China will also incur damage, its about comparative damage. The west runs massive ($600B a year) trade deficit against China right now, since the utility of western currency to China will go to zero under this eventuality, it will be a net loss to China if it actually continue to trade with the west.
Russia is already in a conflict with the west, and Israel more important to America than America itself. China have a lot of button to push it push come to shove.
I think it's easier to just look at this from a larger perspective, once war starts, and a war with Japan is not a small war, the only thing that will matter is real, hard physical scientific, industrial and military power. Most of the basis of western power these days come from tools that only work under the assumption they themselves can stay out of or extricate themselves from any mess they start, the moment that assumption breaks down, the moment when hard power becomes paramount, and it will be in China's interest to make it so, the damage they can suffer can easily explode.
Case in point, a hard embargo against all Asian industrial export, from Mainland China, Korea, Taiwan, Japan and ASEAN, means the west loses access to all semiconductors, electronics and replacement parts, which in turn will not only shut down entirety of their industrial production but also agriculture, transportation, computing, energy, pharmacudicals, healthcare, education, even entertinament, basically everything you can name. Modern civilization runs on what Asia exports, there is no civilization once that's cut.