Vassals are required to pay tribute financially or (more importantly) militarily. Look at Belarus for an example of what that looks like (and the kind of foot-dragging you get when the vassal is unwilling).

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.