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I was in Solitude the second day( in game ) of starting Dawnguard, as I was passing by the little cart market on the way home( it was about 8pm ) I noticed a gentelman derssed in black hooded robes, thought nothing of it just a priest or mage. As we crossed paths I noticed that the finger tips of his gloves where tipped with steel, then I noticed that the steel was sharpened so I look up to his face and saw that this " traveler " was in fact a master vampire. The second our eyes meet he went into DESTROY mode and begain to attack me and the towns folk, the fight lasted a minute or two and the only death was that of the old drunk who hangs out in front of the Winking skever. From this point on I learned a very good lesson, I will make a lap around the entire city, any city , every time I walk in just to make sure there are no vampires.

Once I finished the Dawnguard mian quest the vampire attacks happen less and less and now it's to the point where I only see vampires of any sort out in the wild. So I say that these vampire attacks on towns is just a mechanic of the game to heard you in the direction of finishing the main quest so as to stop the attacks. I really never needed to enter a town while I did the Dawnguard quest and there for never really have that many attacks but they did stop once I had the quest finished.

.... Dots Indicate disreguarded - doesn't concern topic here