There was a dev post a while back talking about making fire vulnerability only apply after using a vampire ability, since fire resistance was nerfed so much that a vampire can't counter it enough to make some pve manageable. I think this is too good, even though all the fighters guild debuffs/skills still apply (pve too, remember dsa)
I think fire vulnerability should be tied to the vampire stages
stage 1 - 10% (could be 0 I guess, but 10% would be manageable for pve trials and such)
stage 2 - 20%
stage 3 - 30%
stage 4 - 40%
I also think stage 1 should be changed to 1 hour.
This makes having the abilities cheaper still retain more of a downside, while also giving more of a reason to feed.