

You'll receive this quest from Sorine Jurard. When you return after completing the task, you'll receive a random enchanted item.

Otherwise you'll have to kill the vampire quietly to avoid incurring a bounty. If you steal the document and show it to the jarl, then you'll receive permission to kill the vampire. He'll tell you that a vampire has insinuated itself as a visiting advisor at a random jarl's court, and he'll ask you to remove the threat. These quests won't really affect anything, but they can give you useful rewards. Prophet, you'll be able to go on several short and mostly random side quests for the Dawnguard.
