My question is:

Are you still attacked by Vampires when you become one? I haven't checked this out as the loss you get in fire resistance is just too great for me to survive.

No one knows as the DLC isn't out yet.

Not the DLC Vampire, the normal one.

Yes. Vampires can be territorial.

Then the answer is yes as Bethesda didn't feel the need to create Vampire Clans like in Morrowind.

You think other vampires would be happy with you recklessly feeding on their prey?

Haha. Even Alva politely kicked me out of her turf.

In regards to the original vampires yes you will still be attacked as in terms the character is clanless and vampires tend to kill off competition in the most basic sense if it can be applied to the game's vampires. So essentialy yes and it may be changed with the dawnguard's type of vampires.