Quote Originally Posted by From a FFVII guide I wrote myself
How to find Vincent: Go to the Shinra Mansion in Nibelheim and locate the safe on the second floor. Enter the code Right 36 Left 10 Right 59 Right 97. A battle with Lost Number will begin. Win and take the Key to the Basement from the safe. Go into the basement and unlock the door at the top of the screen. Talk to the coffin and Vincent will appear. Agree to talk about Sephiroth. Vincent will then return to his coffin and refuse to talk anymore. Upon leaving the basement he will run after you and offer to join the party.