I don't want NPCs to get killed by Vampire Attacks. How do I do this?

If you don't want to mod your game, you can set individual NPCs essential using

setessential <baseid> 1

More info on using that command can be found here: In Skyrim, the console command to set a follower as essential is not working. What am I doing wrong?

Personally, I would go with the mod iber provided. It's not a 100% guarantee that NPCs won't die, but it drastically improves their survival rate, and you don't have to walk around town applying essential commands to every NPC.

I found a mod that stops the vampire attacks on the cities and villages altogether:

No Random Vampire Attacks for Dawnguard (PC)

This is a quick fix for the annoying vampire attacks killing the townspeople in the Dawnguard DLC.

The mod will add an amulet called "Ancient Vampire Repellent". As long as you wear this amulet, the timer for the next vampire attack will continuously set to the far future. When you take the amulet off, the timer is set as if a vampire attack just happened.

V2.0 will also disable the "Traveler"s and the wilderness attacks while wearing the amulet. (The "Traveling Bard"/merchant/etc. and the "Advisor" for the Dawnguard misc. quests are NOT affected by this. The quests work fine. If some of these quests suddenly auto-advance, your target was already killed by NPCs/Dragons/monsters/etc.)

Other options are these mods:

Run For Your Lives (PC) / (Xbox One)

This mod makes most NPCs hide indoors when there's a vampire (or dragon) attack, instead of facing them head-on and possibly getting killed.

A small mod that makes citizens in a village or city run indoors during a dragon or vampire attack.

This was born out of frustration at seeing the citizens all trying to be a hero and rushing headlong to their doom attacking dragons and vampires. Instead, the citizens will run to safety like smart people should when faced with the horrors of giant flying fire breathing lizards (or bloodthirsty undead). NPCs who live in the attack location who own a home there will retreat to it. If the attack location has an inn, NPCs who don't live there will flee to the inn, along with any NPCs who live outdoors. If no inn is present the game picks a random interior from the surrounding area and those NPCs will flee there instead.

This should work for any NPCs added by mods as well, up to a limit of 150 in the area. After that, they're on their own. The NPCs chosen are first come, first serve by the game engine.

Guards, members of The Companions, Vigilants of Stendarr, and the player's followers/teammates will not run, they will stand and fight. Farm animals will not run either, because they're stupid. That includes your horse.
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Vampires will be detected when the events that spawn them begin running.

Protect Your People (PC)

Hate it when NPCs die from pesky dragon/vampire attacks? ...Well boy have I got a mod for you. This mod saves your friendly neighbors from untimely deaths. Technically, it flags certain NPCs Protected.

My mod protects:

• unique named NPCs
• all vanilla children
• named animals - all named dogs, the Stray Dog that can follow you, the goat Lucky from Dragon Bridge, Gleda the Goat, the fox Pumpkin from Southfringe Sanctum, Frost, all Hearthfire pets, Bilgemuck, Old Salty, Lord Tusk
• named creatures - Grok, Moira, Melka, Uderfrykte, Sinding (werewolf)

Timing is Everything - Quest Delay and Timing Control (PC)

Allows you to customize the starting conditions for various quests. You can choose to delay the start of Dragonborn, Dawnguard, or Hearthfire quests, as well as other quest events. Or you can also reduce the level requirements allowing a quest to start earlier than it normally would.
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Unlike the one-off quests, most of the reoccurring world encounters will allow you to adjust their settings at any time. This is the case with the Dawnguard Vampire attacks. Even if you've already begun the Dawnguard quest, you can stop further attacks by setting this value to something higher than your current level. However, I've also included settings to adjust the frequency of the random vampire attacks, which can also be used to prevent further attacks.

Dawnguard: You can stop random vampire attacks from decimating the population, or allow the crisis to build by increasing the difference between the level at which the vampire attacks start and when Dawnguard recruitment begins.

GET No More Dead Merchants (PS4) / (Xbox One)

The Dawnguard DLC is great, but one thing I hate are it's relentless vampire attacks that leave merchants dead in their wake. Once dead, there's no replacing them, causing you to travel more or sell less. Well, no more!

...As your character levels up, the vampires, mages and dragons who raid the towns get tougher, but they remain weak, making it ridiculously difficult to keep them alive. This mod has three solutions for this:

(1) It protects all merchants, so only you can kill them! When their health drops to 1, they drop to one knee, and no further attacks will harm them. But be careful, because your friendly fire can still kill them. The exception to this are essential NPCs, who the game makes invulnerable to keep quests from failing. The mod adds an asterisk next to the name of each protected or essential merchant, to make it easier to recognize them.

(2) It levels all merchants to a maximum level of at least three times their starting level. So they'll level up some as your character levels up, helping them stand their ground against the onslaught of raiders and monsters.

(3) It levels all merchants who are potential followers to a maximum of level 90, so they'll be more helpful as partners when your character reaches higher levels.