Heh, i just spammed those Drain life balls everywhere. You get the same leveling progress regardless of what you kill to get it, same goes with the werewolf.
I actually came across a glitch where the drain life spell would stay in my hand after reverting form; unfortunately it only shows up in the vampire lord's favorite menu so i could not re-equip it if i deselected it.
"We humans love choices." - Mal, Borderlands 2