Yeah, Vincent and Yuffie are the only two secret ones you can use without a gameshark. They've explained Vincent's things, and when you get him, there is a pool of water that leads to a waterfall. You can go in the fall or a cave beside it or something like that and a scene will happen with his ex. It's been so long, but I'm pretty sure you thats where you get the manual to his 4th limit break. Also, by getting yuffie, you can go to the pagoda (don't remember where it's at) and fight your way up the floors and get something else good, but like I said, I don't remember exactly where.