Disclaimer: I play a Templar Vampire.
We all know the current state that PvP is in and the fact that to compete at the top tier it is felt by many that you have to be a Vampire. The real question is, why is it better to be a Vampire than anything else?
The answer that most people give is Mist Form and Bat Swarm. The 60% cost reduction at Level 4 Vampirism combined with armor sets and certain class abilities make those vampires incredibly hard to defeat.
What seems to be ignored however is the fact that problem doesn't necessarily lie with the DK/Vamp or Sorc/Vamp combinations. It doesn't necessarily even lie with the fact Mist Form is a powerful escape tool and Bat Swarm is fairly cheap cost Ultimate with high return in damage and health gain.
The real issue lies with the fact that there is currently no real penalty for staying at Vampirism 4. The only change we see from Vampirism 1 and Vampirism 4 is the scaling reduction in health regeneration. This seems to affect only passive regeneration and has not impact on healing spells or vampirism abilities that provide healing, such as Bat Swarm. Essentially, anyone who plays a Vampire has no incentive to ever give up the 60% cost reduction that Vampirism 4 provides.
So what can we do about it.
Solution 1: Change the scaling health regeneration to healing received. So as they Vampirism level rises the effectiveness of healing decreases. Vampire players would be forced to make the choice on whether the reduced cost is worth a 75% healing reduction and would prevent Vampire players from soloing large groups as their own abilities would not provide the proper healing return. Bat Swarm capped at 6 targets currently returns around 1800 hp. Under this method it would still deal the same amount of damage but would only return 450 healing. Vampire players could still be threats due to the Ultimate, but they would be far more reliant on other players then be the solo juggernauts we've seen in so many VR10 Vampire videos.
Solution 2: Scale fire damage to increase as Vampirism level rises. Starting at 25% and reaching 100% at Vampirism 4. Not the optimal solution as it would not address the Ultimate cost reduction issue, but it would make sitting at Vampirism 4 far more costly in terms of damage taken vs fire sources.
Solution 3: Buff Werewolves. This option would make Werewolves more viable in PvP. Reduce the Ultimate cost, provide them buffs for running in packs (perhaps a passive that adds damage for each Werewolf in the party), or something of that nature that makes solo Vampirism not the only appealing option. Of course the issue with this solution is that we might end up seeing nothing in Cyrodil but packs of Werewolves clashing with Vampires.
Solution 4: Add a new penalty for not feeding. This could be something such as 10% attribute reduction per Vampirism level above 1, reduced movement due to lethargy, or even reduction in magical resistance. This option might allow for a better balance of Vampirism levels similar to Solution #1, though it would most likely end up requiring more time to tweak and balance correctly.
In any event - The primary issue people should be examining is not the individual Vampire skills or even the Class/Vampire combinations, but rather the issue of why there is no penalty for sitting at Vampirism 4 indefinitely.