Giving one creature vamp costs 2dark and like 6 mana. giving ALL of your creatures vamp should cost much more. I'd also downgrade it to bloodthirsty+0, but that might make stuff play too much like zombies. My main concern is that adding bloodthirsty to everything(including your mage, so you may never cast a full action again) will take away too much choice from the player, and that gameplay style already exists in zombies(replace vamp healing with resilient cuz they both make creatures tougher in different ways).
With that said, some other form of limited mass-vamp could be cool. I think an incantation like zombie frenzy could work: full action 8 mana: all friendly living creatures get vamp and bloodthirsty+1.
