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!)
This seems to be a well know bug with using the Save Crystal. This is discusse here, here and here among other places from a Google web search.
Obviously in previous versions for the PC the save game edit should work fine for you (I myself have never experienced this glitch), but if you are having problems with this new version then it is obvious that they have gone through the effort to prevent it for a reason. So even if you do find a workaround, you are probably going to end up with a "hacky" file.
Personally I would go for a restart. Hey, it's FF7 after all, there is always reason for "one more playthrough".
Again, personally, I would have waiting until just before the point of no return to use my save crystal (my first ever playthrough I used it at the end of the first room, and spent hours crying for having to work through so many more battles before dying at Sephiroth - I restarted in the end).
Anyway, I find the crater is generally only good for fast materia upgrading by using the Magic pots. Back in my must-be-level-100 days, I used to use the small islands around the Mideel area for exp growth - there are some sort of green grasshopper type creatures that come in packs of 5 (they are also good for building limit breaks too)
Best Answer
You can recruit Vincent later in the game. Worry not, you'll have a chance to come back! (Of course, you'll miss out on his occasional comment in some story scenes so if you have a backup save you might want to recruit him.)