Here is what i found of vassal behavior in 2.03k
Vassals don't go to war with the same faction when their master goes to war. They are passive. 1 Vassal can have 2 Masters. Some factions hate being vassals and immediately revoked their vassal status resulting in being conquered or accepting vassalage again only to repeat. While other factions are happy to be vassals for ever. If a Faction attacks the Master and Vassal is Allied with Faction, then the Vassal chooses between the Master or the Faction, getting rid of their vassalage if they side with Faction.
Vassal haters:
Pontos - AS
Hayasdan -AS
Bactria - AS
Lusotannan -Roma
Vassal lovers:
Saka - AS
Saba - AS
Naba - AS
Pahlava - AS
Massylii - Qarthadast
I don't much about vassals in ancient times was it to extract tribute, soldiers or to control foreign policy? or am i getting confused with the feudal system?
What does vassal mean in game for the player? When i was playing as Saka Rauka i received a counter-offer to become a vassal of Bactria for 6000mnai after i had offered them money for a ceasefire. It seems I'm just getting paid for an alliance but what else happens?