I'm probably going to get a lot of shit for this, but I can't find Vincent!! I really want to play as him but I just don't know where he is!!! Help me dammit!!!!

He's locked in a coffin in the basement of the Shinra Mansion in Nibleheim. You have to open the safe on the second floor to get the key that opens the door to the room in which Vincent is imprisoned. The combination is:
Right-36
Left-10
Right-59
Right-97
You have 20 seconds to open the safe and you can't pass any of the numbers; if you do, the combination fails. After the safe is opened, you will immediately fight Lost Number.

and then when you're done fighting lost number, go to basement of shinra mansion and go to the room you haven't been in yet, talk to the middle coffin, and do tell vincent about sephiroth... he will be disappointed, and go back to sleep. try to leave the mansion, and before you even get to the stairs, vincent will join your party