Hey Lukas , the vassals now are really boring you can`t do anything with them just ask him for money and that`s all.
Here I have some suggestions for them:
The ability to assimilate his population.
Once the population of the vassals provinces is in minority you can make a referendum and the provinces join to you.
The ability to ask from vassals some army.(like 0.5% of his population per round).
When we are into a war every province conquered by my vassals to give me points not to him.
The ability to change type of his government.
Thank you for reading.