Despite being close to China as the crow flies, the geopolitical difference between Japan and Taiwan means they could be on opposite sides of the globe.
The difference between ally and vassal seems like one of alignment. Right now Japan and the US are quite aligned, but were alignment different, I think the status would be as well.
Japan doesn't get involved in aggressive wars, but neither does e.g. Switzerland, and no-one would call them a vassal for it. Yes that restriction is currently constitutional in Japan, but Japan is free to amend that constitution without needing US approval.