Canon has never given us a direct explanation of what vampires are. Given how they're created they would seem to be a kind of blood parasite that animates the body but leaves the brain largely intact. However, during Angel's trip to Pylea it became clear that the creature within has its own appearance and solid form, it's not merely some kind of demon nanite. So perhaps it is that vampires are a kind of weak shapeshifter who only partially transform the human host when feeding or, sometimes, fighting but who normally find it too difficult to maintain a cohesive solid form. It wouldn't be the only one, since we've seen other sorts of human infections by demons, and in at least some of those cases the demon is destroyed when it is forced out of the human body. Apparently such demons use human bodies to exist within an otherwise incompatible environment.
So perhaps an environmental incompatibility explains why vampires inhabit a human body long-term. However vampires take it much farther than that -- people need to be reminded throughout the Buffy and Angel series that an individual inhabited by a vampire is no longer the same person, because it is often not apparent. As we saw with Sunday's pack of vampires in S4's "The Freshman", vampires often live very much like humans even when no humans are present.
One has to assume there would be an evolutionary advantage to masquerading as a human. The way that vampires hunt suggests what that advantage may be -- vampires can live surrounded by their prey, and they have the benefit of camouflage even when they're gathered together in numbers. While some predators exist mostly in isolation there are others who hunt together in packs and have complex social interactions. There's no reason to think that demons couldn't be the same. In fact, the weaker the predator the more sense it makes that they would band together to hunt.
Although Illyria initially calls vampires half-breeds, she is speaking to Angel and Spike when she does so, two vampires who are indeed two creatures at once. So perhaps more importantly in "Time Bomb" she says of vampires:
"Do you know what you were when I was young? You were the muck at our feet. We called you 'the ooze that eats itself'. You were pretty at night. You sparkled, and you stank. You still stink of it!"
The "you" there seems more likely to refer to the demons themselves and, whatever else her statement indicates, it's pretty clear that vampire demons were very low on the pecking order when it came to demon powers. (The implication that vampires were ooze also suggests that, whatever the form we later see, holding a solid form is difficult for them, hence the usefulness of human bodies). Her statement also suggests that vampires were either in such hunger or were so violent by nature that they mindlessly preyed on one another in their early form and/or multiplied too rapidly to be able to sustain themselves in their environment.
Certainly large numbers of predators hunting together in the same area can be a big problem if the main population group you feed on is neither large nor fast breeding. (Humans are definitely on the high end of the biological scale when it comes to gestation period and time to reach adulthood). But being able to go unnoticed allows for far more success because the hunted group can't quickly recognize the danger in their midst and stays put instead of scattering or fighting back.
It also may have become important because taking on the human form tempered the demon nature even without a soul. Vampires are clearly influenced by the interests and lives of their human hosts: they mimic humans more than other demons. They may also develop the capability for long-range planning, living in hierarchical groups, and engaging in other things besides simply carnage and feeding, largely as a result of the human's hard-wired behavior.
As we've heard at various points, and witnessed during Angel's transformation in Pylea, the demon expresses itself as a fledgling by being dumb and reckless and can't, in its natural state, even communicate with speech. Older vampires are more likely to be cautious and planners, are less likely to give in to hunger and more likely to use violence tactically. They are also adaptable, another human trait. They may feed without killing, either for reasons of convenience or subterfuge (as we saw of Spike's minions in "Harsh Light of Day") or even long-term as a form of survival. As discussed in S5's "Into the Woods":
ANYA: Oh, that's been going on for centuries. Humans hire vampires to feed off them, they, well, you know, they-they get off on the rush.
GILES: And the ... hazards of the underworld can become addictive to ... some people.
XANDER: Why don't the vampires just kill 'em?
ANYA: Because they get cash, hot and cold running blood, and ... they don't leave any corpses behind so they don't get hunted.
As Giles points out, not all vampires can or do stick to the plan, but they have clearly made such plans over the centuries.
This pattern of human adaptation somewhat reverses itself when they become very old. The case of the Master established that the older vampires are, the less human they look and possibly, the less interested they are in blending in with humans. This could be due to the fact that such vampires have become so well established in their surroundings, and so wealthy in the trappings of human power, that they no longer need to rely on camouflage to thrive. The human life loses its appeal to the demon, especially after it's had considerable time to alter the original human mental patterns. It could also be that its ability to manifest itself physically increases as a result of its greater power.
Younger vamps though apparently need to utilize the knowledge and instincts of their human hosts quickly if they're to survive long-term. Assuming that there's minimal higher order thinking of its own, the parasitic demon within takes on all the characteristics of the original human as a result of its memories and established neural pathways. In some ways it is very much the same person, just with very different priorities.
So then what happens when the human soul returns?
The Nature of Souls
What interests me about Spike's early S7 appearance was whether his experience was actually portrayed as being different from Angel's. In Angel S5, the writers hang a lantern on this question with Angel complaining bitterly of the long time he'd struggled with his soul whereas Spike spent a few weeks moaning in a basement and then was fine.
Yet the flashbacks of Angel we've seen post-soul were actually of Angel being either deceptive about his adjustment or simply seeming miserable, not someone incapacitated by the experience. So although he was wrestling with his conscience for a long time it's hard to say if or when he hit some sort of turning point with it.
When S7 begins Spike is being actively tormented by The First which, when it happens to Angel in S3, also makes him hallucinate, seem out of control, and finally suicidal in a relatively short period of time. I don't know as we're ever told how long Spike was alone beneath the school before he was discovered. But it is clear that he's left there mostly on his own for at least a week or more. What's not clear is what his experience would have been like if The First hadn't been present.
I think that, however profound the internal struggle, it wouldn't have been that debilitating for either him or Angel for that long. It's only 2 years since Angel was re-souled that he attempts to rejoins his vampire family in China, having apparently decided that he can function as before despite the shared body he now has. Spike somehow makes his way back across continents to reach Sunnydale within a period of months, or even less, after he is re-souled. The Sunnydale events in the last few episodes of S6 clearly occur within a matter of days whereas Spike would just as clearly have needed to take longer to locate and journey to the cave demons, even if the trials themselves took only a day or two. So although those scenes were interwoven in the episodes, I don't believe they could have been synchronous.
The point being, what effect does the soul actually have once it's returned? My assumption is that the soul is not just some mystical conscience, which is the main thing about it discussed in canon. Rather, it is the consciousness of the human being who was once connected to that physical form. Although for practical purposes in Buffy and Angel the soul has been used as a restraining anchor on the demon, I suspect that some people's souls wouldn't be terribly effective in that regard.
It is also declared, possibly at multiple times in the Buffyverse canon, that demons do not have souls. Yet many are depicted as having consciousness, and some demons are very similar to humans, even capable of interbreeding with them, so why wouldn't they have a soul as well? I'm going to explain it as having to do with a simple difference in terminology about post-death consciousness distribution. Demons automatically go to hell dimensions whereas human souls may undergo a selection process that determines their final destination. (As for human-demon hybrids, who knows? Yet there seemed to be no indication that Cordelia lost her soul as a result of being made part demon nor that Doyle lacked one, so perhaps any human part undergoes the same selection process).
The "why" of the difference in consciousness distribution is trickier. However it would seem to be tied up with some kind of cosmic balance that is central to the stability of universes. The Powers That Be seem to be directly concerned with issues of balance but they're not the only ones who are aware of its effects.
As we hear in S7's "Showtime":
BELJOXA'S EYE: The mystical forces surrounding the chosen line have become irrevocably altered, become unstable, vulnerable.
Athough Giles calls the creator an "oracle" it says quite clearly that
BELJOXA'S EYE: The eye sees not the future, only the truth of the now and before.
So the oracle's power is to see things unseen by others, not prediction. As their discussion deals directly with the issue of Buffy's latest death and resurrection, clearly passage back and forth out of death and life dimensions is problematic and there are restrictions on making a return permanent. For example, while Lilah's seems able to go back and forth at the end of Angel S4, she does not show up again in S5. She doesn't simply leave hell to take on a new existence on our human dimensional plane. There is also another person who has been back from death -- Darla. When she returns in Angel S2, she is fully human.
LINDSEY: Looks like our Darla was a working girl in the New World. Syphilis was what she was dying from when she was human. - Now she's human again. Kind of picking up where she left off. Of course, today something like that could be cleared up with a few antibiotics - if you catch it in time. We're about a month and - what? 400 years too late?
So when her form was restored whatever magic gave her life again began to lose the ability to keep her alive, and the disease that took her life reasserted itself. Presumably had Buffy not died a mystical death the second time, she too would eventually have died again. That mystical factor is also, presumably, the reason she came back with her Slayer power intact instead of simply as a regular human.
So it seems likely that in the Buffyverse, while there are breaches across the realms of life and death, and human and demon dimensions, these are kept to a minimum for the purpose of universal stability. So it could be that there is nothing particularly special about human consciousness versus demon consciousness insofar as it reflects the essence of an individual -- the labeling is all about the sorting process that takes place. But there is a bigger implication when a human consciousness returns from a death dimension in a permanent way.
Continue to Part 3