In the Shinra mansion at Nibbhiem, there is a safe on the top floor. The combination is as follows: Right to 36, Left to 10, Right to 59 then Right to 97. Make sure you don't go to far left or to far right when turning. When it opens, kill the monster and grab the stuff in the safe. Descent the spiral staircase and into the coffin room. Vincent is in one of the coffins. Speak with him and leave, then come back and talk to him. He will join you.