i play with the bizantines... despite the fact that they have a drawback in tech... used carefully and placed well they will kick ass... did the bizantine reconquista of the justinian times by 1250... vassals... bunch of ignorants ingrates who only want protection and do nothing in return... why would i need their money when i can take their cities and burn them down and then make them bizantine and extremely docile to say the least?.... vassals are almost useless... i got one (venice) and it lasted for 5 turns when the french decided that they wanted my lands...
well the main reason to make a faction your vassal is you don't need to train and pay the upkeep costs of the soldiers you'd need to garrison said cities