I've been playing with the idea of a vampire adopted into the OBR hierarchy. The biggest narrative issue is: "Why not build the vampire harder, better, stronger, faster per Ossiarch doctrine?" I can think of a few reasons, first, the area in which the associated legion operates is dominated by vampire superstitions and having an actual vampire in the command structure makes "diplomatic" relations with the surrounding mortals substantially easier and said vampire might even be the progenitor of the very beliefs the legion is leveraging. The next reason would be espionage, vampires can go into mortal settlements with little fear of discovery, whereas the presence of a Bonereaper is hardly subtle, and I'm surprised the Bonereapers even have this blind spot in the first place. Sure, they have their little spy birds, but they're hardly going to be convincing the local noble to sell out his own town, or a guard to open the gates at an opportune time.
And why would a vampire want to join up with the OBR, even with an exemption on being turned into a bone golem? The Soulblight Gravelords faction is rife with internal conflict and, as I've bemoaned many times, armies full of rust and rot. The OBR represent Death's most ordered and productive faction, any vampire that values empire building and tactical acumen over ego stroking would at least have a passing interest in, if not a deep envy of their Ossiarch contemporaries.