I killed a mage in the Dark Brotherhood line, and got kicked out of the Mage's Guild. I need to get 20 Daedra hearts (easy) and 20 portions of vampire dust to get reinstated into the gulid. I have acquired 7 portions of the dust, but I'm not even sure how. I assume I have to kill vampires to get the dust, but where do I find them?

This should really be in the Cheats, Hints, and Spoilers section, but I'll still give you something to work with.