So, I've not looked at Diabolist until today, but looking over it, I certainly like the flavor...
One of the requirements is the ability to cast Shriveling, which is a 1st level Wizard Necromancy spell. This may cause some trouble for your specialization plan, unless you resort to some cheese. Otherwise Knowledge Planes(8) is your gating factor, leaving earliest entry at level 6.
Diabolism: extra d6's against good targets... ok if you're planning on fighting the forces of good. Kind of cool to deal a bit of damage with non-blasting spells. However, very limited uses per day.
Imp Familiar: Ok, you get a familiar even if you dropped yours with a specialization alternate, see the Familiar Handbooks for fun and exciting tricks. So, this guy is an Outsider, which might mean some interesting polymorph tricks, but it advances as a familiar that is keyed off of just the Diabolist level, so it's HD are going to be a problem there...
Vile Diabolism: Here we go. Violate Spell without metamagic cost increase. Keep people from healing. Violate Spell is a +1 adjustment.
Ultimately this seems very weak from an optimization standpoint. Incantatrix, etc. could just as easily pick up Violate Spell and sling around vile spells for free as well. Seems more suited to a DM villain, as vile damage is usually more of a pita for PCs.