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! ;D
I'd also like to know how to change the damage calculations, but that's a different matter...
