Reduced Vassal Border Growth
Reduces the amount of border growth Vassals get per population to 1/3 of its original amount.
Vassals tend to end up with far faster border growth than integrated Regions, since they generate Influence based on population instead of from a very limited number of sources, such as monument buildings.
The trade-off is that low population Vassals will have rather slow border growth when using this mod, though this should balance out once their population grows.