Isran will give you this Vampire-killing order (Gunmar must have been too busy for this one). You’re tasked with locating a Vampire pretending to be a Visiting Advisor to the Jarl of a randomly picked city.
Now; you can go ahead and simply kill the Jarl in full view of the guards etc (but be ready to fight off their onslaught), otherwise, speak with the jarl who’ll ask for proof. Pickpocket the Vampire for his orders and then give it to the Jarl who’ll allow you to kill it without the guards getting in your way. Job done!