I returned to Whiterun and found it overrun with vamps and those vamp black dogs. Some citizens, a few guards and I croaked them all but afterwards the entire town is shutdown. Does this last forever or is it only for a short while? If forever, how can I load and then avoid this?

I tried entering the town night, day or whenever but the vamps are always there.

Make sure you have a save from before you enter the city. If you fast travel, always fast travel to the stables and never at night. That way you can avoid the attacks. Surprised most people don't know about this.

sadly those dead NPC won't get replaced. an oversight I suppose... a DLC shouldn't have such an impact on your regular game.

To prevent attacks, no fast travel. Only travel by day and to locations near the one you want to go to. damn vampires...

I fast travel to the stables. Then I entered the city but I waited until day. Still the vampires attack.

I see no dead bodies after the attack it's just that every merchant remains locked.

Also, as far as I know the city is not supposed to be shut down after the attack?