That would be ok if it didn't leave many places as ghost towns why they didn't have a system in place to replace killed NPCs with new ones is beyond me.

I gave the NPCs a weapon each to help them survive, Including Nazeem. In my characters roleplay he was forging weapons to help the citizens of Whiterun protect themselves.

I make it a priority to kill harkon quickly: once he's dead the vampire attacks in city's stop

My methods are a little time consuming but worth the effort.

1.) I reverse pickpocket better weapons onto the guards of the towns that I visit most.

2.) I always travel with a companion, which for most of the quest line isn't an issue but there are times where Serana isn't scripted to follow you so a regular companion will be needed. They are my second line of defense for the main gate, I will ask them to wait at the main gate to allow more protection as well giving me a little time to react. A second dog or troll companion doesn't hurt either, more the better IMO.

3.) I do a lap or two around the town that I'm visiting, checking every nook and cranny for dust bunnies. The mysterious traveler is the worst and will become hostile the second you get too close. So I try my best to follow him until he's close to a bunch of guards or the main gate area before I engage, this way you can ignore his summons and just focus on him, everyone else will take care of the summons.

4.) The most important one of them all........ALWAYS STAY ON YOUR GUARD!!! The second you slack is the second that they will make their move, catching you off guard. Which will result in the loss of innocent life, tis your duty to ensure safety from the vampires, best take care that you realize this fact. Stay always vigil, never let your guard down, suspect everyone to be a vampire and you will survive.