You can do the Vamp quest on all lvls, you just Need to kill 8 lvl 38 Mobs (? or higher)
Vamp has 4 Stages.
As Vamp you take additinal 40% firedmg.
you have 75% less hp regen (decreast with a passiv at lvl 9)
your skill cost from Vamp skills are decreast
Vamps can sneak 40% faster (or was it 30%)
A other usfull passive is 10% more Magica and Stamina regen
and the Passive undead wich reduce the dmg take when hp drops under 30%
mistform is a good escape skill in pvp
The ulti from Vamp is pretty strong, mostly in pvp.
BUT the additional 40% fire dmg or the weakness vs fighterskill is somthing you have to think about.
Expert Hunter and his Morphs can see Vamps in PVP and deal a lot of additional dmg, ist simply a onehit for you.