What would you think for that for vassalage  ? :
Liege - Vassals treaties :
Each levels of treaties would raise / decrease War support of their people wanting to get their freedom , war support in/de-crease would depends their ideology . Exemple : Liberty axis more war support when treaties get relatives to Influence , Tradition axis same but for religion . You could also for big quantity of gold counter treaties .
Based of war score in surrender terms you could decide witch tier of treaties you will access when your will be his liege .
As Vassals in diplomatic relations if a lower tier treaty is refused he could go to war forcing the treaty they have asked to be signed or if war support difference is enough get freedom but further more treaties are from higher the more war support it will cost for freedom .
But if they lose , the liege would have two choices : upgrading vassalises treaties tier or give a badge ( idk for the name ) to the vassal that will decrease his war support or would restrain him to a max level of war support ( like alliances ) .
Economic :
" Cheap trade " : same as alliance -X war support.
" Shared profits " : gains money based on trade license and commercial roads. +/- 0 war support
" Free access " : get all resources for free , vassals can still trade. +X war support
" Total control " : all ressources are liege's property . ++X war support
Military :
" Shared vision " : Military intelligence is freely shared between the Empires -X war support
" Shared logistics " : same as alliance 0 war support .
" Upkeep share " : vassals pays function of their revenues a part of liege's army upkeep .+ X war support
" Army integration " : with cost of influence / money liege can control like mercenary vassals army for Xturns. ++ X war support
Beliefs :
" Free beliefs " : Vassals can still exerce their influence and religion . -X war support
" Confined beliefs " : Vassals keep their beliefs but can't expand it and if one of liege's territory is converted it would immediately bring it back in his original state ) 0 war support
" Controlled beliefs " The liege empire defines the state religion of their vassals . +X war support
" Cultural control " The vassals influence contributes to extending their Liege' sphere of influence rather than their own. ++X war support
Diplomacy :
" Diplomatic freedom " : The vassals still control their diplomacy , if the two empires were received war declaration they are in war with the attacker . -X war support
" Restrained diplomacy " : The grievances and demands are still handled by vassal , the treaties signed need to be both agreed . 0 war support
" Diplomatic control " : Treaties negotiated with a Liege will be applied to its Vassals as well, and declaring war on a Liege will also drag its Vassals into the same war. On the other hand, grievances and demands caused or received the Vassal must be handled by the Liege. +X war support .
" Assimilated " : Vassals cannot ask for freedom when they go to war with his liege , can't receive support of any other empire ( developed later ) . If liege lost his war , Winner empire cannot ask the transfer of the liege or can't grant freedom to vassal unless they declared war with a grievance they have obtained when this treaty is signed , but it will disappear in 10 turns and will be not triggered unless the treaty broke and re-signed .
Liege-Vassal events .
If the stability of your vassal is two low and in function of their war support independents rebels might cause trouble in your empire .
Or if you're Liege gain fame you could get 5% of his fame ( Interesting to develop it for alliance too )
For technologies a liege could " borrow " some of tech advances boosting the liege for this research , slowing down vassal but he would gain war support .
Vassals-Foreign empires relations .
Official diplomacy would be mostly handle by the liege but other nations could in secret help vassals , as it could be war support creating a scandal if both liege and vassals are in you're sphere of influence , simply gold help or even troops . An empire could also declare war and then grant freedom to vassals and if you don't had sign a new alliance get a grievance if the old liege vassalise again the one you've released . But it would create grievances between you and the liege empire .