There's too much blood in the world anyway!
Raise Vampire works in a manner identical to Summon Spells, except that Raise spells are restricted in that there must be a unit with creatures to raise. What this means is that you must target either a live unit with dead members, or a completely dead unit stack, and that you are limited in the number of creatures you can produce by the number or hit points of fallen creatures from that unit.
If, for example, you can raise twenty vampires per casting, and you cast it on a unit of 30 nightmares that once had 40 members, then you will raise ten vampires (the ten fallen nightmares will be raised as vampires). If the other 30 nightmares are killed, then you can cast the spell on the unit again, and get twenty more vampires, and even once more to get the last ten.
The Necromancy Ward spell will prevent players from raising any creatures at all for the rest of the battle - unless you start removing it from stacks you want to raise.
As with all Summoning spells, the vampires thus created vanish when the battle ends.
Depending on the size of the battle, the vampires you raise could be overwhelmingly powerful, or useless. Vampires have a sort of a 'critical mass' because they can heal themselves with the damage they inflict. If there are enough, relative to the number and power of their opponents, then the vampires will heal almost as much damage as they take, and will be unstoppable. Otherwise the vampires will be destroyed.