Then please share your knowledge of this 'logic' with the rest of us because IMO it clearly, most certainly is an unintended consequence.
First of all the wording of Necromage states: "All spells are more effective against undead." Notice how it doesn't just say "on". Besides this restoration perk is obviously designed for the restoration Turn Undead spellline. But because of the way the buff system works it accidentally also applies to your vampire undead status. There is no indication whatsoever that this was intended on purpose and all the reasons that it was not.
to explain ur logic, ill qoute straight form the manual
"Necromage improves ALL of ur spells , not just Turn Undead spells. Spells with a duration last 50% longer, spells with a magnitude are 25% stronger. So your Destruction Spells now do 25% more damage to undead."
So being ALL spells last longer and magnitude are stronger against undead, when ur a vampire and the game counts u as undead all spells that u cast "against" urself are gonna be stronger. the game is labeling u as undead so since u can cast spells still on urself, the game is counting those spells as against an undead whereas enemy u could only cast harmful spells but with it being ALL spells and u can cast on urself, the ALL spells means ALL spells. And form reading that, yes it does tend to side with this is what part of its purpose was, weither intentially or unintentially, the perk is doing whats its suppose to do.
to the OP, great job again finding this out, makes me wonder what else is in the game that all us have yet to realize right under our noses.