Japan's constitution prohibits a traditional military as a result of surrender in WW2. Not having the right to solve your own disputes sounds like vassalization to me. From a face saving perspective "powerful ally country" is probably the status of every vassal state. From a de facto perspective they are not allowed to solve their own disputes and house military bases of a country that solves disputes (with the threat of violence) on their behalf.

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.