It also is free to amend and revise its constitution, to terminate the US-Japan defense treaty (Article 10), to pursue its own foreign policy goals, etc. "Vassal state" is an old term with specific connotations that fits Japan if you sort of squint the right way but really doesn't.
To military conflict? I doubt it. To political and economic conflict? Probably. The same thing would be true of France or Poland. That's generally what happens when an ally country stops being an ally country, but it doesn't imply that they're a vassal state.
France and Poland are also US vassals lmao. France can make a better case but that’s because it admitted that all of these alliances subserve it to the US in the 60s and has fought hard to retain some semblance of independence.
