Good question... I have invested exactly zero into any form of magic, skills, perks or magicka on my vampire thief/assassin.
So... yeah, I'm wondering about this too.
I was kind of assuming the vampire lord powers would be their own thing (seeing as they have their own perk tree no?) and not tied to any existing magic skills.
But if they cost magicka to use... I may have to rethink my approach.