Well then, as you may or may not know Dawnguard adds random Master Vampire assualts on cities, villages, and small farms around level 10 (though they may begin earlier). Supposedly, these attacks are like the random dragon attacks, but more lethal especially if there are two Master Vampires attacking the city.
So to make sure you do not end up with an cities, villages, or farms I am attempting to compile a list of ways to prevent this. Feel free to add any other tips!

1. Be very careful of fast travelling. Like dragon attacks, vampire attacks chances increase if you fast travel and arive at NIGHT (7pm - 5am).
2. When entering a city at night, stay around the main gate for a few minutes. Usually the vampires will tend to attack soon after you enter. If you do not wait you increase the chances of the cities populace being killed.
3. Villages and farms are a bit trickier, the vampires appear to spawn outside of the villages/farms, but only when the player is near.
4. When in doubt, lower the difficulty! Although this will also make the NPCs in the village die even faster, usually it is usually worth doing.
5. If you are level 10, or around there, use Flames or any Fire enchanted weapon you may have. Even if you are not a Destruction mage, flames SHOULD prove somewhat useful against these vampires. When combined with novice difficulty this may make the fights shorter and with less causalites.
6. If you have Healing Hands use it on the NPCs that are fighting the Vampires. WARNING: Some NPCs may view Healing Hands as an attack!