You could argue about vassel-ness, but yes, the whole Western block would, and many poor nations that lean that way as well, like Vietnam or Palau.
Vietnam has some beefs with China, but I think it’s a bit more up in the air than that which way they’d turn since it’d be a real existential threat to them to militarily oppose China like that (plus they do have some productive deals with China).
True, and I have no special insight into Vietnamese politics. In this scenario the US is also right there, though, and is stronger than China assuming no MAD.
Vietnam and Palau should be read as examples only.