I've been wondering this for awhile, seeing as at various times the Vampire skill line has been overwhelming with a number of different builds (and Werewolf still sucks).
So why isn't there a skill line that all players get as long as they are not a Vampire or Werewolf?
Clearly in PvP, in a lot of situations Vampires have an advantage in survivability, damage, stealth speed, etc. The fire damage vulnerability isn't cutting it because of the fire resist enchants and the damage shields/sword and board that most vampires run.
Try laying down an AOE like NB Refreshing Path on a vampire and see how many absorb messages you get.
It's the only way I see to balance them, as clearly ZOS isn't going to lock vampires out of the Dark Brotherhood or the Thieve's Guild (and the passives from those guilds will make them even more powerful than us mere mortals). That is, balance them without nerfing the line.
Or does ZOS expect everyone to roll a vampire or werewolf?