In Cloud's hometown, there is a safe in the mansion. In the safe, you are required to ender in a code at the right time (very tedious as well). Combination is RIGHT TO 36, LEFT TO 10, RIGHT TO 59, RIGHT TO 97. After the code is inserted, you need to fight a boss. The boss is tough if you're vastly underleveled. Make sure your best mage has good cure spells packs and make sure your Limit Breaks are to the point.
After you've defeated the boss, I recall you get a new Materia (forgot what it was). Enter to the cell halfway right next to Sephiroth's library, you'll see Vincent in a coffin. Vincent will ask you questions, and you'd have to check the coffin a few times. He'll refuse at first, but when you attempt to leave, he'll change his mind and decide to join you.
Have fun with Vincent.