What I propose is that we remove the ability of the victors to impose an implicit vassalage out of which there is almost no escape, without naming it, and substitute that for en explicit vassalage that can be escaped from given enough time.
I don't think your system allows that to happen. It does not change the fact that:
1) Players don't *want* to be vassals, so won't choose the vassalage option
2) Players often want to be very punitive, and impose extremely insulting and unpalatable conditions on defeated foes
3) Players find it almost impossible to accept these extremely punitive conditions
Instituting an option that garnishes X% of a realm's income to send to another realm as tribute does not seem to me to be something that will change any of those facts. If Realm A can't take all of Realm B's lands, then it can drive them rogue, and then replace Realm B with Realms C and D (as happened with Ibladesh).
