My point was that what you describe does not require any new mechanics at all. There are already limitations to realm sizes. There is already a mechanic by which a realm can transfer gold to another one (send a noble to the central realm capital and transfer the goal). There are already mechanics for one realm to send troops to help in another realm's war (alliance). And there are already cases where these mechanics are used in a relation that is hard to describe in other terms than as a vassal-liege relation.
The existing mechanics are so onerous that it discourages anyone from doing such a thing. If we don't see such tributes take place, it's not because nobody would like to impose a tax on the defeated foes, it's because managing such tributes are just too much work and unlikely to pay off.
