Not everything you kill provides, in loot, what it appears to have. There's a reason why Vampires don't get crossbows, don't think you can 'twist' the system and still get crossbows. General knowledge.
You CAN use crossbows as a vampire....you can see it clearly in the E3 footage. And what is general knowledge? It should be common sense that any weapon flagged as "playable" will be able to be used no matter what. Cross bows are flagged playable, obviously. You kill and enemy and take their gear. They wouldn't be made unlootable to vampires specifically, that would be an inconceivably dumb integration.
That is like saying vampires can't already make ebony weapons. Dragonbone is just a new weapon type being added into the preexisting dragon smithing.