Now, onto that roleplaying element...
Bonereapers have personalities, and their own motives, but they really don't have any needs to speak of. They need repair, sure, but they exist in an undead stasis if left to their own devices. The Mortisans search for knowledge and perfect their necromantic artistry, the Lieges administer their armies and cities and refine their military arts. They are made for a purpose and pursue the mastery of that purpose, but the society of the Bonereapers lacks a certain vitality. They are characterful instruments of Nagash's will, and that is one of the reasons I wasn't keen on playing them at first.
Vampires, however, are free agents, with needs and desires that can be contradictory or self defeating to their Nagash intended purpose. They also allow the undead to have nuance and to interact with mortals in more ways than just viewing them as "prospective fodder." Obviously, a vampire that trucks with the OBR is of a more conformist breed, but where a Bonereaper would mercilessly reap the tithe, a Soulblight might find or indulge in other means of satisfaction.
The problem with monoliths is that while impressive, they are ultimately dull once the initial shock has worn through. As I've mentioned elsewhere, it surprises me that the OBR don't take more advantage of their undead cousins in warfare, which leaves the faction sort of one dimensional. The Bonereapers are a machine and it's hard to step into the shoes of a machine. And GW agrees with me, the interesting lore and books about them come from edge case personalities.
So, how does this work? Easy, the Mortisans find a vampire with sympathetic values and of sure loyalty, and make him a Liege over a legion in an area requiring the talents and charisma only a vampire can possess. The OBR keep their secrets and the vampire holds power over a fighting force by which all other pale. He's not perfect, especially by OBR standards, and his survival in his current form is based on his merit and continued success. The stakes are high, the rewards are great, and the possibilities flow like the sweetest bloodwine.