Isnt payment of annual tribute almost a necessity? Most German campaigns into Bohemia, Poland, & Hungary were partly for this reason.
In the late middle ages, the Hapsburgs, Luxemburgs, etc., sought direct rule of their eastern neighbors, but earlier (900s-1273; Saxons, Salians, Staufen), the main goal was just to force them to acknowledge imperial overlordship, pay annual tribute, and sometimes send contingents for imperial campaigns.
The Kings of Poland, Hungary, and Bohemia, unlike the other imperial vassals (dukes, margraves, etc.), had sovereign status in their own realms (which were never considered part of the Kingdom of Germany), and were just "tributaries" that owed the Emperor a vague allegiance.
So if you cant rule them directly or make them normal vassals as part of your own kingdom, then how else would you simulate this kind of relationship in the game?
Could this also apply to Wales, Ireland, & Scotland? Again, only in the late period did the English Kings seek to gain direct rule; before that, their status as pertains to England was something like that of the above kingdoms, right?