I've just started a new game (due to my HDD dying and taking with it all my saves).

Basically, I had done Helgen, gone to Whiterun, got the Golden Claw quest, headed off and done that, came back to Whiterun, got told of the dragon at the Western Watchtower, and was just leaving to go there when a guard says to me "They say if a vampire just scratches you, you get infected". I finished the first dragon encounter and see the Jarl, am just leaving Whiterun to go camp somewhere, this being around 10.30 pm, when a master vampire strikes.

I don't recall any trigger incident, it just seems that Dawnguard activated straight up....although, that said, Hearthfire kicked off early as well, with the Jarl of Falkreath offering me a title, housecarl, and land for only doing the first dragon encounter (he must be needy?!?).

I actually had to reload from earlier saves because after waiting through the night, I left Whiterun to go buy a horse and found two deathhounds had wiped out the stables...bit rude I thought seeing in game terms it was only three days or so since Helgen.