I see what you are saying, but also, Japan can do things to reverse a number of those restrictions, some of which are functionally self-imposed, can't it?

And even for the ones that exist as a matter of international agreement and treaty, it's hardly like the US is going to seek compliance via force. If anything, at this point, there must be a non-negligible amount of support in American foreign policy circles for Japan's resuming a more independent and militarily capable stature to help deter China.

If this was like 1950 and you said Japan was a vassal, that would seem reasonable, but I think at this point the situation is too complicated to put in those terms. For example, you could also say America is playing some kind of mercenary role and Japan is kind of in the very civilized and posh position of being able to pay another country for bodyguard services. That probably never worked out for a country or state in the long-run, but that's not really the point; one might call such a country weak, but not really a "vassal".

You just keep checking more boxes into what fits the description of a vassal state.

Being a vassal state has benefits. Protection being one of them. The Japanese people are smart, they saw their options and realized being a vassal state of the US (and not the Soviet Union) was in their best interest.