I've beat Dawnguard once already, but I deleted it cause the Vampire attacks really pissed me off.

If I download it again, should I expect the same Vampire attacks or have they been fixed? I don't mind them while I am doing Dawnguard, but it got annoying when I was trying to do other quests and they attacked too often.

There wasn't anything to fix...They may have turned down the frequency of attacks, may have.

Finish Dawnguard first or be more careful about when and how you enter the city.