Do Vampire attacks begin at level 10 all the time, or only after Durak asks you to join and the quest 'Dawnguard' begins?

Is it at all possible to prevent the Vampire attacks?

People have said that at level 10 that's when vampire attacks start. I don't know for sure though as I don't have characters at the moment that low of a level.

I would like to know, too. Any mods to work around it? or any word from beth?

Vampire attacks do occur before level 10.

No, you can't prevent them from occuring, but you can stop them.

Important tip: do not travel near cities (cell) at night, if you can. It will save a lot of NPCs.

Moreover, keep an eye for a mysterious man with glowing eyes walking inside any city at a certain point in the game; he is a vampire.