I was thinking of it from the perspective that Europe still has, albeit worthless, an army. They have a some autonomy to do things the US doesn't agree with or doesn't support.

Japan had their constitution written by the US. SK's constitution states that in times of war, the military's command will be handed to the US.

Those are very blatant vassal traits. Economic exploitation is one thing, but the US would legally turn these countries into Ukraine when time comes