The question seems to be about whether the stat mods themselves can be edited, rather than how to change which of the mods Vincent/the materia use.
Unfortunately, I don't think these things are known, since they are almost certainly hard-coded into the game. However, I'd very much like to know and edit them.
If anyone does how how they may be changed, do speak up!
I'd also like to know how to change the damage calculations, but that's a different matter...
