Eeee... I'm not sure that's possible to do with the base Vampiric Aura ability. You can do it with triggers by running an action on units that have "vampiric aura" buff.
Now, I know some people like to edit Vampiric Aura into an ability that only effects the wielder. Do you need the actual aura, or do you just want one specific unit?
If you only need one unit, you can use the "Item - Life Steal" ability. That works on ranged units.