So, if you download Dawnguard, and even if you don't pursue the first quest, there's still vampires going about killing NPCs? Or does this only happen after you complete the initial quest or two? This is what I'm not clear about.
As soon as either almost every guard you pass mentions they're opening a Dawnguard, or an orc named Durak approaches you. It can start from any point after that.
In my experience there won't be a vampire attack every time you enter a town. So far at level 36 and still not having go to join either side, they're kinda rare.
But when they do happen, they can be very annoying. Adrianne kept getting killed. Had to reload 3 times before deciding to fast travel somewhere else, waiting a few hours, and then fast travel to Whiterun.
They still attacked, but Adrianne was safe in her home.
The only way to stop the attacks are to either kill Lord Valkihar at the end of the Dawnguard side, or I assume joining his side will stop it sooner. But Dawnguard has much better goodies than the vampires.
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