"On entire other sides of the globe" is a pretty strong statement. Japan is a vassal state of the US and Taiwan's independence from China relies wholly on US support. Chinese citizens (at least the ones I questioned) have no love for Japanese. There are even talks of a Pacific NATO between Vietnam, Korea, Japan, and the Phillipines, with Taiwan being the clear Ukraine parallel. South Korea is probably politically like Germany (primarily self interested), Japan is probably politically like the UK (sees the bigger picture).
Japan is by no means a Vassal states. It has the third largest GDP in the world, and is a major player in its own right, although you are correct in asserting that it is militarily comparable with far less wealthy nations.
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.
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.
Japan is free to change their constitution at any time. AFAIK they are no longer bound by WWII-era treaties with the US. (Can anyone confirm/refute that? Google fails me.)
Japan at least have recently changed a few laws to increase their military forces. Japan is a free country and they could change treaties if they wanted. We are just better off working together. I wonder what would have happened if say China & Russia didn't exist. Then there's be no one to threaten Japan, make them want to be a buddy of the US.
