We know of at least three human souls that were brought back from death and re-united with the physical body a vampire had taken over -- Angel, Darla, and Spike. As mentioned earlier Darla returned fully human, so the demon who had killed her did not get pulled back. Instead, Darla was revamped later by Dru.
One question I have about vampires is whether they are all the same demon (essentially, clones) or if each demon is somehow unique apart from its human host. It does not seem likely, for example, that the same demon who was killed in Buffy S1 is somehow restored to Darla's body once she's revamped in Angel S2. But if the vampire parasite just creates clones of itself when it finds a human
host, the question becomes somewhat irrelevant. Darla's human brain retained the memory of its life as a demon because the physical form was restored from its point of destruction (in Buffy S1) but with the original human inhabitant restored from its demise in 1609.
Once Darla is revamped, the new demon parasite steps in to take over her body and mind just as it would with any other human host. (It's also possible, since Drusilla is Darla's second sire, that the demon is largely the same because it comes from the same line as the original. But the possible relevance of vampire lineage is a discussion for another day!)
However in Angel and Spike's cases, their bodies are not simply swapped out by first a demon, then a human, then demon again. Rather, the body becomes shared by both human and demon, something which would inevitably, one would imagine, lead to a power struggle. While it is not one where the human is able to expel the demon, it does seem to be one where the human seems to have the upper hand.
There is, no doubt, a strong resistance to the occurrence, particularly with Angel when it has yet to be voluntary. Although Spike never speaks of himself in a dual way, Angel does on a number of occasions, quite aside from the literal difference in his name. One of the earliest is in S2's "The Dark Age" when Angel gets infected with Eyghon but forces it out of his body:
ANGEL: I've had a demon inside me for a couple hundred years... just waitin' for a good fight.
BUFFY: Winner and still champion.
Presumably part of the reason the battle with Eyghon was so short (though it didn't look particularly pleasant) is that both human and demon who already shared the body were both in agreement that they wanted the intruder out.
However it does make sense to me that the human is more likely to be able to control the demon if it wants to. The human is the natural resident of that form, even if the demon has had control of it for a long time. Moreover, we've already established that the vampire demon is naturally predisposed to following habits and impulses the human has developed, and that it's more likely to give form to desires the human has repressed than to introduce entirely new ones. For example, the killing of one's family is common for new vampires -- and even among humans the most likely people to kill us are our families or others we are close to.
Nevertheless it strikes me as a risky proposition by the Romani to have resouled Angel as a punishment. Without having known the original human host, they were taking a chance that he hadn't already been a killer in his human days. On the other hand there was little doubt that having a shared body would lead to some kind of internal dissent, and, given that he was already a mass murderer of some renown perhaps they considered it a small risk.
Spike, on the other hand, knew quite well what sort of person he had been and would have had at least some idea of what to expect his human's reaction to be to everything his body had done since his death. So if he did want to change, gaining a soul could have been expected to work.
Balance and Stress Fractures
Earlier I discussed the possibility that any transition back from a death dimension is either unable to succeed on a permanent basis, or carries serious repercussions should it succeed. I think the key clue for why the balance is important lies in Angel's return from hell in S3.
In S7 Beljoxa's Eye talks about the The First's return as connected with Buffy's return to life. It's in S3 when we are introduced to The First and Angel is targeted. In S7, it is Spike who is manipulated. These appear to be deliberate connections, so my hunch is that the maintenance of "balance" in the distribution of souls between heaven and hell, and life and death, is important in keeping The First in check.
BELJOXA'S EYE: It cannot be fought, it cannot be killed. The First Evil has been and always will be. Since before the universe was born, long after there is nothing else, it will go on.
Yet for such an eternal entity, The First seems relatively powerless in the human sphere or else, given Buffy's line of work, we should be running into it all the time. Instead, its appearances are all tied to people who have defied life and death. And we have another instance of the importance of the dimensional barriers in S6's "After Life":
WILLOW: Think of it like, the world doesn't like you getting something for free, and we asked for this huge gift. Buffy. A-and so the world said, 'fine, but if you have that, you have to take this too.' And it made the demon...I think it's out of phase with this dimension. Like, its consciousness is here, but, but its body is caught in the ether between existing and not existing.
The idea of balance is expressed here by the actual creation of an entity to restore the balance of Buffy's life moving back across life and death dimensions. Not only that but note the separation of body and consciousness that Willow points out. I think the issue of physical transference across dimensions is also important, as this entity seems to have piggybacked on Buffy's reanimation but is unable to manifest in its own form. And we should remember that when Angel both was sent to hell and brought back form it, both the demon and human consciousness and the physical form made the trip. So breaking that barrier, especially in a solid form, seems to create a weakening in that barrier which centers around the entity that went back and forth.
While Buffy's first death in S1 was enough to disrupt the Slayer line, it was not enough to trigger The First. This isn't surprising since her death was brief and not unlike what many humans have experienced over time. Her return in S6 was very different. It was neither a typical human death, nor was it brief, and both the human consciousness AND body were restored, just like Angel's in S3.
I think this balance was first upset when a human soul was brought back to life to reside in Angel's body during his initial re-souling. While Sahjhan's meddling in Angel S3 indicates that prophecies can't always be counted on, there are a number of them revolving around Angel. His direct dealings with the Powers suggest that he is a key player for some reason. That reason might have developed because his re-souling was a chink in the wall between life, death, and the transfer back and forth of souls. While a small matter at first, its increasing mobility may have created a form of "stress fracture" over time.
Also from "After Life":
WILLOW: The demon. I-it's gonna dissipate. The only way for it to survive on this plane is if it were to kill the subject of the original spell.
It is Buffy's physical presence in the human plane that makes it possible for the demon to exist at all, it is tied to her. I think that this is equally true of Angel in "Amends" making him vulnerable to direct communication with The First.
Although in that episode Angel believes the First has returned him from hell there's nothing that confirms this idea. The only thing we get from the First is this statement when Angel decides to die.
Jenny: You're not supposed to die. This isn't the plan. But it'll do.
Why would it be equally useful to The First for Angel to die? We know that The First wants to turn Angel back to its side and particularly to kill Buffy. Both of those are understandable goals for an evil entity. Perhaps it's because Angel's physical form and human consciousness being in hell was destabilizing, enough so that he was restored to life. So if instability is a favorable thing for The First, then it would prefer for him to return there so as to further weaken that barrier.
My hunch is that Angel's significance to a future apocalypse (as believed by Wolfram & Hart) has to do with the constant loss and restitution of his soul. This happens once on Buffy but twice more on Angel, in the S1 episode "Eternity" and again in the S4 arc ending with "Orpheus." I think that this repeated movement of Angel's soul makes him a critical stress point in the effort to maintain balance and hold back The First.
Return of The First
After The Powers act at the end of "Amends", The First appears to retreat, only to re-emerge in S7 when it once again tortures a vampire in hopes of having it either kill Buffy or be killed itself.
In Spike's case, the body and demon consciousness hadn't gone anywhere, but yet another human soul was moved from death back to life. And this time The First could not only affect Spike directly but also began to appear to a number of people and was able to carry out a series of systematic attacks through its Bringers.
Beljoxa's Eye doesn't mention Spike, but the connection seems obvious nonetheless. The transfer of another soul back from death, especially after such a considerable period of time, is destabilizing. After all, Buffy has been alive again for some time before The First reappears. It seems that it is only once Spike is re-souled that its campaigns begin, perhaps because a sort of critical tipping point had occurred in terms of The First's ability to operate.
Which brings us back to the mysterious cave demons and their role in the human plane, and my hypothesis that they are, specifically, creatures with the power of transformation. Another way to express that was that they are demons of metamorphosis, that is a change of form or shape, especially by witchcraft which might have, in the Buffyverse, a more central role in the order of their universe.
Another entity who seems clued into this activity is Skip, the demon we meet in Angel S3. He claims to work for the Powers That Be and tells Cordelia that:
Skip: "Life and death, that sort of thing, they got a handle on. Who someone chooses to love, well, that's just good old free will. See Cordelia, the visions are an ancient, powerful force. Demons are the only ones who can withstand them."
Whereas Spike and Angel are dual entities because the human was returned, Cordelia becomes part demon without losing her human side. Exploring how this affected events in Angel S4 is a long enough discussion that I'll postpone it for another time or speculation in comments. But Cordelia's transformation at the end of "Birthday" seems less likely to be the cause of imbalance as her eventual movement from one dimension to another in "Tomorrow." Skip implies to Angel in S4's "Inside Out" that her transformation and movement across dimensions facilitated the birth of Jasmine. Whether it was really Skip pulling the strings on that plan or not, it seems that he was at least able to create some powerful transformative changes and breach dimensional barriers. Is he related to the cave demons? There's no evidence of it, but they are apparently empowered to do similar things. I'll only say that Cordelia's two seasons of suffering through the visions could be seen as their own trial, and the importance to her of continuing as a seer prompted her request to Skip for a transformation.
In Ovid's The Metamorphoses, the first story is about the creation of the universe, in which chaos changes into order. In the Buffyverse, the maintenance of dimensional balance is critical for keeping chaos from erupting. The significance of the physical body as a locus for dimensional weakness (as seen with Angel, Buffy, and Cordelia), and the transfer back and forth of souls (as seen in them and Spike) does seem to have some relevance in the events of S7 as well as S4 of Angel. What's more, Cordelia, Angel and Spike also hold a larger symbolic meaning when these dual-consciousness bodies and dual identities are seen as the site of ongoing struggles between the same good and evil forces at play in the larger field of the Buffyverse.