Once you arrive at Shinra Manor, you get hints around the mansion specifying some numbers. In the old days, that was quite the trial. Nowadays, not so much. Once you find the safe in the left corner of the mansion, you must put the numbers in the proper order and without exceeding the value while inputting the commands. Right to 36, Left to 10, Right to 59, and Right to 97. The safe will open, and you will face a boss called Lost Number, which can be pretty hard if faced unprepared. After defeating him, you get some items and the key to the basement. When you unlock the basement and check the coffin, you find Vincent. As you are leaving the basement, he will join your party.