Come Thou forth, and follow Me: and make all Spirits subject unto Me so that every Spirit of the Firmament, and of the Ether: upon the Earth and under the Earth: on dry land, or in the Water: of whirling Air or of rushing Fire: and every Spell and Scourge of God, may be obedient unto me!
-The Bornless Ritual
Vulthoom, to hear him tell it, came to Mordheim after narrowly escaping an attempt by his mentor to kill him. The reason for this is not clear, but given Vulthoom's penchant for skullduggery it is likely related to precious items which suddenly went "missing." That being said, Vulthoom has also told the story in the reverse: that it was he who tried to kill his mentor.
Either way, Vulthoom's knowledge of Necromancy is not complete. Possibly due to his shady past - or, perhaps not.