I tried playing as a Vamp but I can't do it. Only thing beneficial about being a Vampire is the added Frost Resistance and no Health Regen during the daytime.
That and Shriekwind Bastion are good places to get it early on.
Necromage perk. It enhances everything the game considers a spell effect on you. This means potions, enchants, shouts, even some perks.
A vampire illusionist is very powerful. A vampire necromage is probably the most powerful build in the game.
A small example:
My shield of Solitude gives 30% magic resistance and 35% increased block, thats what the item description says.
Until I equip it and the necromage comes in to play, because under active effects it says 37% magic resistance and 43% increased block.