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. 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.