Hello !
I read somewhere on another forum ( i think from Skyrim's steam forum ) that there is an issue about this expansion that when you start dawnguard quest , vampires start to attack cities and wipe out npc's .. is real that ? if you dont strictly follow the quest when you trigered it , vampires start to kill Skyrim's populace? That was an annoying thing to read about this expansion and made me think twice about purchasing it ...otherwise i consider it just trolling ...

Vampires start to attack cities between the hours of 8pm, and 7am, after Dawnguard is installed, but stop after you beat the Dawnguard MQ.

as long as you dont go outside during the night, and do the MQ quickly, you shouldn't have any NPCs die.

Your best bet is to not travel directly into a city. Just fast travel to the stables. If you get there during the night, just hit the wait button until 8 AM and then enter the city.

If you enter a city around midnight through like 4 AM, then you can initiate a vampire attack and have NPCs wiped out.

Ok thank you very much .. that makes sense , but this " rush " the MQ quickly is making me anxious and uncomfortable as usually i prefer to have time for my quests .. I understand that " doing the MQ quick " means to end it as soon as possible.. without doing other quests meanwhile.. and as i fast travel often sometimes i have nt the sense of time , night or day..

Oh god!!! That sounds too tricky for me ... !!!!

If you start the main quest for the Dawnguard, and have already started doing quests to kill vampires, will the vampires still attack cities?

The vampires still attack after you complete DG main quest. however they are severeley lessened, and bareley happen outside of an eclispe. And the vampire attacks are exagerated, so dont worry. I tested it out by fast travelling directly to cities and villages multiple times. Even at night, i only encountered one toatal, and that was after i entered and exited a builidng at night. I then was able to fight them off with the guards relativeley easily. A Master Vampire and two Death Hounds. No casualties. And im a level 39.
That said, they do happen, and are dangerous, so be careful.