Would this work?
Mount a Vamp on a Hellsteed, put him in a unit of Hexwraiths, so the Hexwraiths can still move through a unit as the Vamp flies over it.
Mount a Vamp on a Hellsteed, put him in a unit of Hexwraiths, so the Hexwraiths can still move through a unit as the Vamp flies over it.