Is Vincent recruitable during the Nibelheim flashback?
Solution 1:
It is not possible to open the door to Vincent's tomb during the Nibelheim flashback. You'll have to wait until you actually arrive in Nibelheim for realsies to recruit Mr. Valentine. (Even if it was possible, hey, it's a flashback, so he wouldn't magically appear anyway!)
Solution 2:
No, you need to be able to fight the mansion boss (the one in the safe) in order to get vincent. Only after that battle can he be recruited, and when you enter the shinra mansion in the flashback you aren't able to fight any enemies or search for the clues to the safe.