Are you on the PC? I'm sure by now they've made mods to do the job. On the 360 you happen to be screwed.
I play the xbox360, so no help from mods. Thanks for the idea though.
You can do side quests at any time, like with other questlines, vampires can attack at any time, just like a vampire character you are playing can wander around at any time. No idea why you would get attacks before level 10, had you done something to start the Dawnguard questline? You can I think start Dawnguard before level 10, but you would have to go looking to start it before you got the hints from the guards or the recruiter found your character. My newest character is still happily ignoring Dawnguard despite getting all the hints, he told the recruiter he was not interested and no attacks have happened yet.
I was walking toward the city gates and passed a guard and he told me about vampires getting out of control. I thought this was strange because I wasn't level 10, and at that moment that stupid vampire appeared and attacked the guards. Could that guard have initiated this? It would be impossible to avoid guards. What level are you? Yeah you can start it before level 10 by going to the dawnguard fort.
I used an earlier save from when I was level 8 to go to that fort with the intention to complete that main quest, but it's quite a challenge on that level. I don't know what to do as apparently npc still get attacked after you have finished the main quest... Also you can't avoid all npc from getting killed as some attacks happen at a different part of town from where you are, sometimes you arrive too late and sometimes you don't find out there was an attack until you find a body or notice a merchant never opens his shop again. I just don't want to start a new character only to visit ghost towns when I'm level 50 or so. What to do, what to do... Did you take some precautions Aussie500? You do not always get the recruiter, sometimes guards tell you about vampires and sometimes you get a vampire attack to start dawnguard.