There a few things to take into account. Vincent's is what you could consider a magic user. His stats lean towards Magic, as a result, you probably want to equip strong green or red materia on him.
HP: Very Low (-5.2%)
MP: High ( 4.6%)
Strength: Very Low (-11%)
Vitality: Low (-9.1%)
Magic: Above Avg ( 2.8%)
Spirit: Above Avg ( 4%)
Dexterity: Very Low (-10.1%)
Luck: Below Avg (-3.9%)
You mention that you have Vincent's ultimate weapon, which is Death Penalty. This gun's power increases every time Vincent kills an enemy, using this formula:
Power = [Power * [Vincent's Kills / 128] / 16] + 10
So eventually, he could become really strong if you put some effort into it.
Additionally, there is a well known glitch that causes Vincent to kill enemies with one shot, provided the Death Penalty's power has been boosted to an extreme value. More information can be found here.
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
The item Cornucopia cures small. It's cheap at 150 gil and can easily be bought in bulk so you never run out. Wearing the Ribbon accessory also fixes this easily.