This could be a hero class, like the Death Knight, where a quest chain allows for the creation of a new class. This class would be available to Blood Elves, Orcs, Undead, Humans, Night Elves and Gnomes as these races would make the best vampires  Also there would be equality (3 on each faction).

The vampire would be average in both melee and magical attacks, if a bit focused towards the magic side. Spells, instead of costing mana, would instead cost health. This would be a radically different approach to playing the game, constantly having to balance firepower with survivability.

The Vampire would be a mail user, allowing it to make use of hunter and shaman gear, and would be able to use Swords, Axes, Daggers and Fist weapons (bladed weapons suiting the role well).

Unique moves would include Bite and Blood Drain, draining the health of other characters in different amounts.
Neck Rupture would be a DoT, lasting 10 seconds initially but increases with talents.
Bat Form would transform the Vampire into a bat, basically a flying mount, much like the Druid's Crow form.
Blood Rush would buff the Vampire's party, much like a Pali buff, but with a weak HoT spell or +damage buff depending on the Vampire's spec.

The three talent trees would be tailored to the different play styles of players in World of Warcraft.

The first talent tree, Decay, would be tailored towards the PvP players. It would favour aggression, and would give players more damage for their health reductions.

The second would be the PvE or raiding spec, called Crypt. It would enhance the previously available spells but have more health loss overall. This is because the player would be better supported in a group.

The third would be a compromise, called Undeath. It would offer new abilities and more survivability options for solo play.

The Vampire could also have a passive class ability which gave the player a buff at night or when fighting indoors (due to the low amount of light).