How is it meaningless? Japan was conquered and has had its constitution written by the US while hosting a large number of its troops. The only difference between this and any other historical vassal state is the US paints an illusion of not being one and its economic system doesn’t require tribute to profit greatly as it favors trade.
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.
