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I have a Khajiit stam NB Vamp. I love it for PVP. With the sneak speed bonus and double take I can move around the battlefield very fast in stealth, and rip off some big hits from stealth. You could do the same without Vamp with the Night's Silence set, however you sacrifice some damage since you can't use Hundings Rage. Vamp can excel in PVP, especially at end game when you have some CP to mitigate fire and Dawnbreaker damage.
I would not recommend doing it for the regen. You have to have a skill slotted for the regen. The abilities are all next to useless on a stam build except for maybe Bat Swarm. A NB has a ton of useful abilities that are better than vamp abilities, so you may have trouble finding a place on your bar for vamp abilties.
I would strongly encourage you to NOT go vampire if you plan on running alot of group dungeons. There is just too much fire everywhere, and vamp makes you too squishy and a liability for you group. I have a different toon for dungeon DPS, because I die too much with my vamp NB.
Haven't tried Werewolf. I would say that morphing into Werewolf is far from an "I Win" button, and usually when I see the average person morph into Werewolf in PVP they are quickly targeted and cut down. That being said, some people can build some really powerful builds with Werewolf and Molag Kena, but it takes some planning.