I'm thinking they attack anywhere, even if the player isn't present. I say this because I started a new game and after following Hadvar back to Riverwood the first thing I did was clear out Bleak Falls Barrow for the golden claw and dragonstone. By this time it was about 10pm in-game so I found a bedroll and slept inside the Barrow until morning. When I woke up I went right back to Riverwood and tried to do the fake letter to Camilla quest but Faendal was nowhere to be found. He's disappeared from the game. Not in Riverwood, not in his house and no corpse anywhere.
So I went to Whiterun and killed a dragon and blabla, slept for another night and in the morning went back to Riverwood. Faendal is still nowhere to be found and now Sven is dead next to a cart. I wasn't in Riverwood at night at all, I never slept there at night I slept first in Bleak Falls and then the next night in Whiterun, so it was obviously attacked while I wasn't there.
Which is completely ridiculous. The way I play the game, I never fast travel, I roleplay, I don't like to rush, so if the Dawnguard quest begins at level 10, that means vampires are going to be attacking NPC's, including quest-giving ones until I'm level 10, then they're going to be attacking places all throughout the quest which will take me a while to do... Why the hell does this expansion destroy vanilla content? That makes no bloody sense to me whatsoever.
