BtVS by Whedon and Mutant Enemy. Shadowrun RPG by FASA, WizKids, FanPro, Hare-Brained Schemes.
Borrowing the demons as soul-stuff concept from Piers Anthony's Xanth.
-Cleveland, 2000 AD, Buffyverse-
This reality was ruled and fought over by True Demons until, in ancient times, humans and other things rebelled, exiling their former masters to less savory dimensions. Lesser demons still roam the world, along with human magic-users, but modern society does not believe.
The last True Demon, in fear of being forgotten completely, created vampires to carry on its legacy. These creatures were able to kill humans and reanimate their bodies with lesser copies of the original Demon. These risen vampires possess all the memories of their human lives, but their human soul has fled.
While being made of similar stuff to the human soul, and existing on the same intangible plane, these demons burn out too much of their energies on keeping a foothold in the physical world to act as a proper soul. As a result, the vampires they animate have no conscience.
They can, however, feel.
There's a dive in Cleveland known as the Side Bar. They don't serve blood, human or otherwise. Too much overhead and the owner doesn't have the resources for a steady supply. It's a common hang-out for the winged-and-horned crowd; just about anyone can come in and be served a drink as long as their money is green, or a color not too far from it.
One particular night, a black-haired vampire took a seat at the bar. He'd been in enough times that the owner poured a shot of whiskey without having to be asked.
The next customer in the door was a nondescript brown-haired vampire. He paid for a drink and took a seat in a dark corner. Black-hair noted that the name in brown-hair's wallet was Sam, but otherwise ignored his fellow creature of the night.
About an hour later, under the influence of the alcohol, black-hair was pouring his heart out to a bald guy in a leather jacket. The new guy had introduced himself as the 'Mailer Daemon' and had said that he was enjoying one of his few days away from his computer.
Black-hair had mostly ignored this info, in favor of his own sorrows.
In fact, the depressed vampire was nearly at the point of tears, thoughts of all the stuff he missed from being alive spilling out of his mouth...
The torrent finally ended in the sentence: "I wish I could feel my heart beating again."
The Mailer Daemon specialized in things like subtle dimensional transference, although certain things like direct teleportation were beyond him. This wish was within his power to grant, even while somewhat drunk, so he did.
-Seattle, 2063 AD, Shadowrun-
A few dimensions to the reft of the 'verse, there lies a much more magically-capable reality. Modern society has no choice but to believe in things that go bump in the night, as they made national news fifty years ago. Magic in this reality is caused by the nearness of still other half-formed realities. When those 'metaplanes' fade into the distance, so does magic and the creatures that depend on it.
When magic is at its peak, the dimensional barriers are the weakest. In the past, this has allowed unspeakable Horrors to consume the surface of the planet, causing the collapse of civilization many times over, but they are the ones most affected by the ebb and flow of magic.
While the next natural conjunction of realities is still hundreds, if not thousands, of years away, many advance scouts of the Horrors have taken strides to hasten the coming Apocalypse. They've been warned of the immense leaps in human technology and for the first time, in the vision of a future Earth fully prepared for their arrival, the dark hordes have found something to fear.
The vampires of this reality are not a purposeful creation, but the natural result of the Human Meta-Human Vampiric Virus, a rapidly mutating strain evolved to incorporate ambient magic into its genetic code. While its victims enter into a state of near-death, they do not actually die. Souls are not forced out and replaced by a foreign entity; instead they are as much of a victim of the virus as the body.
The souls of these vampires require the Essence of other souls to remain intact, but they still are, on a very basic level, human.
Also, while one of the core themes of this reality is that its native magic can't alter time or open portals to stable alternate worlds, by the same token there's nothing to prevent other realities where such things are common from reaching in and taking things out...
Quietly managing to ignore the daylight, a viral vampire was sitting on a bench in a park, waiting for her contact to arrive. She was planning to hire a team of mercenaries to take out her sire without letting them know about her true nature.
Between partial shade, the sunglasses and the immensely high SPF sunblock sufficiently advanced tech had brought to the world, she was able to withstand the daylight-
Right up until the point her heart suddenly stopped beating.
She had a second to ponder the anomaly, before even the indirect sunlight overwhelmed her and she burst into flame.
Mere seconds after her first scream rang through the park there was nothing left of her but a smoking pile of dust.
Left in her place was the red-eyed, green-skinned, spiky true form of the creature that had been pulled from the demonic kind of vampire sitting in the Side Bar.
The demon laughed as it felt its arms and leg because, normally, it was impossible for its kind to exist separate from an ex-human host.
However, this new world was a place of vital magic and a truly stable astral plane...
It crowed with glee as its physical form faded from the park bench to exist in the realm of light and shadow, a plane that most mortals could never hope to see… A truly awesome place to hunt from.
It zoomed off in search of victims-
Only to have its head bitten off a few seconds later, a thousand miles away, by a giant preying mantis who had decided She didn't need the competition.
-Side Bar, Cleveland, Buffyverse-
"Granted," the Mailer Daemon announced solemnly.
"Yeah, right," the changed vampire said, crossly, finishing his drink.
He didn't notice his heart beating until he had drained the last drop of whiskey.
By then it was too late; the second the liquor hit the new metabolism of his stomach he was overwhelmed by nausea.
He was barely able to make it out the door in time.
The bemused Daemon moved to follow the black-haired vampire, only to have someone walk up from behind, place a well-manicured hand on his shoulder and say: "... I wish to feel my heart beating again."
The Daemon turned to meet the gaze of the brown-haired vamp, 'Sam' by his wallet.
After staring into those brown eyes and the unsaid thoughts flitting behind them, the Daemon came to a decision and handed the undead being a small brass ring.
"Help yourself," the granter of wishes stated, before he turned around, swaying slightly, and followed the first vampire out the door.
Uncertain of what had just happened, Sam gingerly held the circlet up to the bar's mounted light and, very gently, touched the pulsing darkness within.
-The Previous Week, A Dark Alley-
He'd taken the wrong route through town and was now paying the very unexpected consequences.
Unable to fight off his attacker any longer, Samuel Verne felt a sudden pressure in his chest, the pain second only to the fangs in his neck as the vampire behind him drank deeply.
Figures, he thought darkly, a heart attack on top of everything else.
Too soon after that, he was drained and left for dead in that alley.
All too soon he rose from the dead, a nightmare preying on his friends and family...
Shivering after the momentary contact with his past self, Sam the fledgling vampire pulled his hand back, his fingers wet with blood.
He licked it off as new memories flooded his mind and he knew that… somehow… he had not only touched his still-beating heart, but his human soul as well.
After a pause, he came to a decision - there in the town where he had died.
He held the circlet up and spent his last few moments 'alive' calculating the angles from the presumed placement of his old heart to that of his sire, who, all things considered, had turned out to be a rather large jerk. Once he was certain of his path, Sam forcefully plunged his arm into the hole through time.
-The Past's Dark Alley-
It would be hard to tell which of the two men was the most surprised when the arm erupted from Samuel Verne's back and plunged into the chest of his would-be sire.
The supernaturally strong hand clutched that vampire's heart and ripped it out, causing both it and the surprised undead to explode in a cloud of dust.
Its work done, the displaced limb did not withdraw back into Sam's chest. Instead, it simply ceased to exist.
Sam, now unsupported by the vampire's grip, sank to his knees, slowly bleeding from the wounds in his neck and the gaping hole in his back.
The hole which led all the way down to his obliterated heart.
Strangely enough, he was filled with a sense of immense accomplishment as he toppled forward and the lights went out for the last time.
-Outside the Side Bar-
Quite unaware of Sam's fate, the altered black-haired vampire ignored the Daemon standing beside him as he sprawled on the ground, the last feeling of nausea fading from his system to be replaced by a hunger, a craving, and a realization.
He breathed without effort. His heart beat. His inner demon was gone, replaced by his own personal soul.
However, his hunger for blood was still there along with a new craving for something... else.
The black-haired vampire stood up and walked away from the bar, towards the darker section of town. He walked down a few twisty alleyways (some half-existing) and arrived at the door of the Pink Porsche, the local demon brothel.
A regular customer there, he was granted admittance to the Bar & Grill portion. He ordered an overpriced vial of blood, fresh from one of the staff and was sated surprisingly quickly, yet his new craving only grew stronger…
Leaving that place, letting his feet take him where they may, he found himself outside an apartment that he could tell by smell was home to only a healthy female.
In the dark night, he stood outside her house and pushed, not only against any supposed barrier to his intrusion, but against his very being. Somehow he was able to disrupt the bonds that held his molecules together. With the new changes to his body, it was child's play for him to slip underneath the door.
He re-formed in a living room where a huge lacquered cross decorated the wall.
He reached out and touched it, yet it did not respond to him. It would not burn him, for there was no longer a demon living within him.
There is pain, he thought. But is of the mind, not the body; it is... what do you call it... psychosomatic. Still, Ouch. He removed his hand from the cross and winced, rubbing his reddened flesh against his leg. However 'real' the pain was, it still hurt.
He strolled into the bedroom and saw the woman there, peaceful and beautiful. He perceived her soul, her essence, her life, swirling within/throughout her flesh. He very gently put his hand onto her shoulder and she woke, eyes wide with fright.
Amidst her scream, he sunk his teeth into her neck and drank of her very soul, made fresh and full with her fear, along with a token amount of blood...
The world of demons and Slayers would never be the same. The changes seen in him aren't half of what the causal agent can do. HMHVV isn't just a spell or a magical being, it's a virus... And viruses evolve very quickly.