I am trying to peg the challenge rating for a villain in Libris Mortis.
The creature is described as an Ogre Magi Vampire with 7 levels in Necromancer.
I imagine you would apply the template to the Ogre Magi and then add the levels in Wizard as Nonassociated class levels?
I am thinking CR 15 but I also could see CR 14 because he loses his Regeneration which I would think is a major part of an Ogre magi's CR 8.
Actually in the Libris Mortis that vampre Ogre-mage still has his regeneration listed (Which I feel was an error, as regeneration clearly states that creatures with out Con scores can not have regeneration, but which is not corrected by the Libris Mortis Eratta).
you are correct that the wizard levels would be non-associative... so the 7 levels would add 4.5 to the CR (I generaly round up when adding non assoiciative levels to monsters so 5).... when the 2 from the vampire template is added it comes to 15...
however if you remove the regeneration aspect (Which I agree that you should, undead with regeneration are just broken) then lowering his CR to 14 looks like a good option.
Yeah I just looked at the errata today and was suprised they didn't fix the Ogre mage/Vampire in the adventure. Fast healing plus Regeneration is pretty brutal, unfortunately my DM used the creature as written in our adventure.
Thanks for the comments thats pretty much what I was thinking for the CR.