To get Vincent, the player must have reached the Shinra Mansion in Nibelheim. Vincent is behind the locked door in the basement, and the key can only be obtained by cracking a safe on the second floor.
Four numbers are needed to open the safe, but they are scattered about the mansion. In a corner of the foyer, the player can find a letter containing hints to their locations. The safe combination numbers are hidden in a chest in the atrium, behind the piano on the first floor, in a hallway on the second floor and printed on the letter itself in invisible ink.
Once these numbers are successfully entered into the safe on the second floor, the player must then defeat Lost Number. Lost Number moves very quickly, but it is possible to use Stop and Paralysis on him. Lost Number has two separate sides, and when one is defeated, the other side will gain very high resistance to the type of attack that delivered the killing blow.
Once Lost Number is defeated, a key is found in the safe. This key unlocks the door to the basement, where Vincent sleeps in a coffin. Speak to him about Sephiroth, then speak to him a second time. It will appear he is not interested in joining the party, but as the player leaves the basement, he will change his mind and catch up.