A couple of tips:
1. I found starting with a vampire easiest. Get him/her to the power level you want, with the abilities and weaknesses you want, 'cause you can't change 'em once you hybrid into a spellcaster.
2. Delete your dark form. You can't access it once you hybridize, and it creates a new, random one every time you use an ability. So unless you are a chaos vamp, this gets annoying really fast. You can edit their normal appearance as normal.
3. Watch that Charge! You can't see what your charge meter is when your a hybrid, so be careful. The discharge ability and a familiar are kind of a must here.
A couple of other notes. All of your vamp and witch abilities seem to work, but you CAN lose the context menu options for Vampire movement (travel as bats, teleport as mist, etc...) when you click on the ground to move, but you still have access to them if you click on your sim.