Right now, Masters tithe and prevent their vassal from attacking them (excepting during the Red Horseman, of course)... and those with an Embassy also force their vassals to vote similarly.
Why not add a couple of other restrictions, perhaps with new buildings for their effect? To encourage vassalizing, as vassals have been lacking lately?
1. Ability to move opponent to an adjacent (and visible to you) Sea of Rust space... this would cost you 5 Coal and 1 EUR, as normal, but move the vassal, instead. This could allow you to move your vassal closer to you (if they're two or three spaces away) or to move them further away (if they're one space away).
2. Ability to prevent vassal from doing research. Perhaps this would require an Embassy while you're a Technocracy, or something. This wouldn't prevent the vassal from stealing TLs from others, just prevent the Academy one...
3. Ability to "rebuild" the vassal - basically lets you build buildings for them. Any kind you can build. This way, you can rebuild them up with Warehouses just before they are unvassalized so you can nuke them again XDDDD Well, more like you can apologize for that... I dunno.
4. Maybe the ability to give your vassal to someone else?
5. Maybe the ability to give your vassal items in your inventory directly. So you can dump all your useless coal on them, so they'll overflow it XD
Just tossing out ideas, but I really want the ability to force move.