Out of curiosity, I played an entire game the other day on Coward and Very Easy mode, just to see if I could convince a vassal to agree to a peaceful annexation. This was using the IGC. In 300 years, having spent literally tens of thousands of Ducats relationship building, I was unable to convince any of my 3 vassals to annex. This leaves me wondering whether there should be a setting somewhere to improve the probablity of a peaceful annexation.