Both Werewolf and Vampire are powerful creatures. They both have strengths and weaknesses. There is 1 key difference, a Vampire will always be affected by its weaknesses because the Vampire effects are always active, whereas the Werewolf will only be affect by its weaknesses while he is transformed. Therefore you will see less Vampires than Werewolves in ESO simply because Vampire has too many downsides to it. However, with the right build you can also make up for that as shown here with the Nightblade Vampire Build or the Necromancer Vampire Build.