To get Vincent, all you need to do is go to Nibelheim and enter the Shinra Mansion. Go to the room with the big safe and enter this combination: Right 36, Left 10, Right 59, Right 97. Make sure you save before you enter the combination because when you do, you'll have to fight a boss that can be pretty hard. When you beat him you get the basement key and the Odin Materia. Now go to the base ment and open the door you couldn't earlier. Talk to Vincent about Sephiroth then talk to him again, and as you leave the mansion, he'll join your party.