When I first down loaded DG, Iwas using a high level character, started the DC quest and the attacks started. I started a new character but have not started the DG quests, and have had 2 minor attacks. Now that is not proof that you won't have them, but with the first character I had a LOT. Also, fast traveling seems to trigger them, whether you actually see them or not. You may come back to a city and find that some NPCs are dead. IF I fast travel I always run around looking to see if any vamps have spawned (because they don't have to spawn by you), and check again before I leave....