If you want to open that safe there are some steps in early game must be done. 1. You need to help rebels. 2. You need to let the theives guild leader to go from jail. Then you will have open vault door and a Rugald at courtyard party. After - you can find the 4 animal mosaic in the castle - and open the safe. If you give a letter from safe to Rugald - you will help Rosalind to become the new duchess.