Edit: Just experimented and found out that perks taken whilst being a necromage vampire are boosted by 1.25% too! So the damage perks are increased, all the perks that increase by a specific value (e.g. Increase skill effectiveness by 20%) are boosted.
Te illusion perks that affect how high a level the spells work up to are affected also, some destruction spells damage are increased (not entirely sure how this works, some spells are effected and some aren't), so my flames spell does 27 damage per seconds instead of the 22 it should be (illusion fear perk and +50%).
Wow, excellent bonus discovery! I didn't do further testing after discovering this the first time to figure out if future perks would be affected.
So to be a more powerful damage dealer, one would attempt to boost restoration FIRST to get to the necromage perk and vampirism. Then start up the damage trees of whatever you choose. I wonder how well that affects the destruction tree's damage. Aspect of Terror would then add 12.5 to each fire cast, and the two destruction perks would add 62.5% to fire spells. Also, magicka reduction perks would be affected, allowing you to more easily reduce magic school magicka requirements, each producing 62.5% off instead of 50%.
Interesting. I may have to restart my mage at some point.
That would also increase the level bonus of the illusion perks, right? I wonder if it also affects the slow-motion speeds of perks like in archery. 50% slower becomes 62.5% slower? Would also affect smithing and enchanting perks?
This discovery that it affects later perks really increases the usefulness of this combo and in what order you get them in.