I think the space vampire was both a bit of fun (in an otherwise bleak novel), but also one of the explorations of different types of mind/consciousness; the mission has a the narrator (with half a brain), a vampire, a person with 4 consciousnesses residing in one brain, a massively-augmented person, a combat specialist who controls a vast set of external peripherals, and an AI (and, of course, the aliens). They're there to make you consider different aspects of what makes up consciousness.