The bit in the dev journal a while back about spinning off parts of your territory as vassal/allies got me thinking -- lack of vassals was one of my main gripes im some other games.
Presumably vassals are almost like alles, but the vassal automatically votes for the overlord in any vote to determine planetary ruler, and his territory counts as part of the overlord's for determining when the overlord has enough to end the game. Also, probably the vassal is automatically at war with anyone who is at war with the overlord, but not the reverse. Vassals can have their own private little wars. Lots of room to tweak the details, this isn't isn't supposed to be a comprehensive list.
Ideally both alliances and vassal/overlord terms can include gifts & tribute. Depending on my power and mood, I might tell an independent faction...
A: become my vassal and I will give you 'x' amount of some resource every turn forever (I am rich and trying to buy my way to the throne)
B: become my vassal and pay annual tribute of some resource you have lots of and in return I will give you an annual subsidy of something I have lots of.
C: become my vassal and pay annual tribute or I will squish you like a bug.
When I spin off some of my territory as a vassal, I can determine the starting terms, and they can't do anything about it except rebel. But favorable terms may rebellion really unlikely.
Then, ideally, a human player can become the vassal of an AI faction, at least until it is convenient to rebel
Also, when/if anyone has time to get fancy with starting conditions, some factions could start really powerful, with some vassals in their pocket on turn1, and it should be possible for a human to start off as a vassal.