I think vassals should be as dynamic as they are in CKII. In CKII Vassals can declare independence. That'd be awesome, but it shouldn't be without rhyme or reason. You should be able to dynamically set tribute for each of your vassals. Low tribute will(naturally) get you less money but it would also keep that vassal happy and another side-effect will have more money to spend on their military making them good allies in a war. High tribute will increase the likelihood of their declaring independence. Also if you suffer a major loss in a war(i.e. lose territory or lose a large percentage of your army), have high WE and negative stability and high revolt risk then that increases the likelihood of vassals declaring independence, but there's got to be really low relations first.