Vampire Form gives +15 Stam Regen/+3 Mana Regen. It makes you immune to up to Greater Poison as well, DP has a chance to punch through the Poison Resistance, and Lethal always will. You also get 20% Life Drain with weapon attacks.
Vampire Form used to be pretty good for Necro/Mages, you wouldn't take damage from spells that required Garlic if you used a Scroll, Arcane Clothing or LRC for it. Then they made it to where using a spell that required Garlic (Even if you used Scroll, Arcane Clothing or LRC) does 17-23 damage to you. You also take 17-23 Damage from drinking a Cure Potion, since they're made out of Garlic.
So basically, a Necro/Mage in Vampire Form can only heal with SS, if he tries to use a Mini Heal, he'd end up taking more damage than he heals, and if he uses Greater Heal, he'd only heal 20s damage, if he tries to Cure himself, he ends up damaging himself. Vampire Form is basically for Warriors now, and actual Necro/Warriors are kind of rare, especially on some shards.