Quote Originally Posted by ToriJ View Post
Yuffie and Vincent are bonus characters which you don't need in order to complete the game. To recruit Yuffie you need to pick the right dialog choices when you find her or she'll run off with your stuff and Vincent is concealed in something or whatever in Cloud's hometown inside that Shinra building and I think you need to know a combination or something. Can't remember, been too long and I don't feel like googling.
You get Vincent by knowing the safe combination (gives you some nice items, one of which is the key to the basement) then going into the room and having a chat with him.