
These are my ideas for vassals:
1: Nation bonus claims should be given in percentages every week or so, to prevent nations vassalizing for claims, then releasing after 10 minutes. For example, a vassal and their overlord could get 25% of their vassal/overlords total nation bonus every week that they've been vassalized for. If the vassal is released or revolts (and wins freedom) this progress gets reset.
2: For similar reasons to #1, nations should also be vassals for so long (2 weeks seems reasonable) before being able to join their vassal/overlord's sieges.
3: Vassal's SoI is given to their overlord nation, similar to how occupying a fort gives their SoI to their occupier.
4: Vassals should already be inside of their overlords SoI (or at least connected), so that vassals are an extension of their overlord's territory, instead of a colony (this is so that nations don't vassal to one another to join sieges halfway across the world).
5: Overlord nations should have access to more taxation, such as income tax, a percentage of their vassals occupation tax (if the vassal occupies any towns) and resource tax (I'm not sure if this is already a thing, but it should be).