2. They could lure and capture Dawnguard scouts/hunters, killing all but 1, vampirizing that sole survivor and having them share information and location of the Dawnguard base, then send them back to make it easy to enter the base!!
3. before all that, make underground multi floor labyrinth underneath Vampire Castle, in case the Dawnguard find their base first. So then, when Dawnguard are spotted, they would be lured under the castle, so they would have to go through all the traps and ambushes before they could reach the leader, who is actually the dragonborn behind the curtain to surprise and eliminate the intruders and prevent their escape (and vampirize any that surrender).
4. go to a secret mages hideout, and vampirize them, and vampirize other cults/factions to spread the Vampires influence and numbers, so that the Dawnguard can be overwhelmed and crushed!! So this #4 plan is a great example of strength in numbers!!
So what do you think? The Vampires have a cool castle, they have vampirism to their advantage, they have vampire powers, they can boost their numbers, they can spread their influence faster than the Dawnguard. And infiltrating the Dawnguard is easier than the Dawnguard trying to infiltrate the Vampires.

http://www.youtube.com/watch?v=CIZ-o5VlhCo