Do you have to have triggered normal dragons (Whiterun Western Watchtower) in order to really activate Dawnguard, or just activate the mod? I ask because I'm not sure I want to deal with random vampire attacks in the cities just yet.

Once installed the DLC self-activates once you reach level 10. That's when you'll be approached by an orc named Durak to inform you of the Dawnguard recruitment drive. Also that's when the vampire attacks in cities begin to occur.

The quest may start by overhearing dialogue from city guards once you are level 10 or, as mentioned above, by being approached by Durak. You can also begin at level 1 by traveling to Fort Dawnguard on your own.

Vampire attacks begin at level 8.

Ah, I had thought it was level 10 before the attacks began. Thanks for the correction.