when i create or download mods for dawnguard, they don't work
new content is there, but anything that modify dawnguard content wont work

i made a mod that add a new armor and make the 2 dawnguard dogs essential
the armor is there but the dogs can still be killed

i also try to remove weakness to fire on vampire by deleting all spell and magic effect with "weakness to fire" name
but when i choose to accept to become a vampire, i still get the weakness to fire

same thing with the mod "Dawnguard no Vampire Attacks" http://skyrim.nexusmods.com/mods/21615, the amulet is there but vampires continue to attack towns

Have you tested using a NEW game?

(or test using coc from the main menu?)

It sounds like a saved-game problem ...