As a general rule, Pokemon are banned as a whole rather than banning individual aspects of them. If we were to take that route, it could potentially lead to problems like "Only SD sets of Scyther are broken, so we ban Swords Dance!" (not the best example but you get the point). Barring exceptional circumstances, we can't "nerf" Pokemon that should be banned for the sake of "diversity."
Vulpix is banned because it imbalances the metagame. Moody is not banned under the same logic as Vulpix, instead it is banned for the same reason we ban evasion moves, brightpowder, etc: It introduces a significant luck factor to the game, regardless of what Pokemon it's on. The two cases are incomparable.
Twas bryllyg, and ye slythy toves
Did gyre and gymble in ye wabe:
All mimsy were ye borogoves;
And ye mome raths outgrabe.