Go to ShinRa mansion in Nibelheim (not in Kalm Flashback) then you do this puzzle where you can obtain the numbers for opening the safe (dont remember them right now), then you fight a boss (Lost Number I think), then you can get the Key to the basement, Nanaki's last limit manual, and Odin... go to the basement, open the door, check the coffin and talk to Vincent (there are two options in between)... He'll go back to sleep... when you get back to the stairways, he will join you...