Why don't you think about what you've said here.
You made a MISTAKE in the way you're playing The Elder Scrolls: Skyrim and for that MISTAKE of going into a town while having Dawnguard installed we're going to kill off some of your shop keepers and misc quest givers.
You shouldn't have to be tiptoeing around something as ridiculous as this to keep content in your game available for a character. A lot of people don't like this feature and it just isn't something you can ignore in game. You have to go out of your way to avoid NPC deaths at higher levels, whether it's constantly saving/loading or avoiding settlements entirely.
I only ever reloaded two attacks to avoid the consequences, and one was not even a vampire attack. It would have been three reloads if I had realised one attack had occurred but it took ages for the courier to catch up to my character with the bad news, I could not be bothered going back that far just to save two NPC's in Riften I did not need. They were the only two NPC's lost permanently due to vampire attacks. Only my first character had any problems, the second one learned from his mistakes. One reload was a dragon attack on Riften, caused by the same silly mistake that lost the first two NPC's. Having lost two I was not going to lose any more and reloaded, first to try and beat the dragon with no fatalities, then I gave up and just went straight to Riften instead of going to the stables, which avoided the problem. The second reload was something I had no way of knowing would happen, the vampires spawned a long way up the road, I waited, nothing appeared I thought it was safe. Stuffed it up and lost an NPC while being stuck in a conversation. Reloaded a few times, ended up charging up the road and killed the sods before they got to the town. My first two characters were high level ones, not specialising in magic. They seemed to always have enough time to get rid of the problem before any NPC fatalities happened (although most of my NPC's were well armed and stocked with health potions), it was only not being ready for the attack that caused problems. The dragon was a hopeless situation, he was determined to kill NPC's.
I admit in the end my first character decided avoiding towns was the best idea, but he had already had 6 odd attacks by then, and the only walled town he missed getting an attack for was Solitude. He had no further reason to visit towns, he was just dropping off all that extra loot. Having finished most of the side quests he is currently waiting to test out if anything with fangs is planning on attacking him on his next town visit. My second character was still doing smaller quests while doing Dawngard, but being a better fighter had no problems knocking off her horde of fanged invaders. I never once needed to reload any of her town attacks, all she lost was one guard, who was quickly replaced.
So no you do not have to constantly reload just to get through Dawnguard. Some might, but most should be able to easily manage a bit of a fight. Does no harm to save just in case though.