I've read that vampire attacks during the main quest can wipe out every city life form, and that finishing tha MQ stops them. I'd like to buy the DLC, but I'd also like to save the NPCs, so what level should my PC be to do the MQ from start to end (dawnguard side) without stopping by?

The Vampire town attacks are the easiest thing in the world to manage. All you have to do is not Fast Travel to a town or city so you get there at night, do not enter a town or city at night, and if you are in a building in a town, just don't go outside at night. Which is what you would do if there were Vampires about.

If you need to travel to a town, just save first. If you arrive after dark, then reload, wait a bit of time, then travel again. The DG MQ did not take me to many cities, so vamps were easily avoided.

If you want to have a town attack, then wait in a house until 6 PM, then until 8 PM, then until 10 PM and then go outside. This will get all the commuters to go home so mostly what is out to get attacked are generic guards.

Ty for the explaination, it's useful! Still, I'd like to know if there's a recommended level for the MQ, so I can start it at the appropriate time: I don't want to start it at lvl 1 and get in places full of big mean vampires only to have to postpone it: one thing is taking care of dailight and do it at my pace, another is starting it and being unable to finish it for like an ingame month, and having to put up with vampires, dragons and such at the same time

The Vampire attacks start at Level 10. The game will not even invite you to the new content until then. So, that is the minimum level according to the Devs. The enemies are scaled, so they should be matched to your character. There is one fight where it gets real tricky, but you have help.

Ok, then 8 more levels to go on my new character!

Oh and when you do get it please dont start yet another thread about "these vampire attacks are ridiculous"

No, don't worry! I'll ask for a mod to stop them in the mod section!

Although lvl 10 is the "official" starting point, I always get my characters to around lvl 20 before tackling it. But thats me!
Don't believe all the horror stories about vamp attacks either, but never fast travel direct to towns. Pick a spot nearby and walk in(before dark)