I posted this as a comment to bitterbyrden in loligo's kickass entry (which reminds us that we cannot trust the characters to know the full picture of what the hell's going on metaphysically in the Jossverse) -- but as I hadn't really seen anyone put forth this thought yet, figured I might as well post it here too.
Regarding the soul being what damns Spike (and Angel) to Hell, per Pava(y?)ne, and the relative fairness thereof. I wonder if it's not that the soul damns them to hell per se (as if being aware of the wrongness of one's past actions, done when one couldn't appreciate their wrongness would make one suddenly deserve to be punished for them upon becoming aware, but not before), but that the soul (in Pavane's view) damns them to have an existence after their vampiric death. And it's just obvious that they're going to Hell (as it's obvious to Gunn, Wes, Angel... in their own subjective views).
So, most vampires would just disappear forever on being dusted. Demon doesn't go to hell, it just unexists. Thus Darla never seeming to have any memories even when she's turned into a vampire again of existing (or being just "nowhere") anywhere after she was first dusted, though she remembers being a vampire, even before she's re-vamped. Her body was recreated, with her brain intact and all of the human and vampire memories engraved on it still there, but no apparent memory of heaven for the human soul, or hell for the vampire demon.
(What this means about where her human soul was after she died the first time, I'm not sure -- perhaps that it was in limbo, prehaps that everybody's afterlife is different and Darla just didn't remember hers? It certainly doesn't discount there being an afterlife for normal human souls, since we have Buffy as potential evidence for that. Though we don't *know* that Buffy was in Heavennnnnnnnnnn, as loligo's entry should remind us. We have only her subjective memories of where she *might* have been.)
But for vampires, having the soul added to their human brain/demon spirit combo means that the brain/demon/soul amalgam becomes susceptible to the laws of the Jossverse afterlife. Which includes going somewhere after your physical body is destroyed. So, not "fair" to the soul, if the soul is a thinking, feeling entity in its own right, but if the vampire's soul is just an extra metaphysical spark that gives them a conscience and the ability to make choices based on morality instead of expediency (especially if it's not the same human soul they had when alive, though Lurky's "We will give you back your soul" makes this seem questionable) then the moment the vamp!soul is united with the brain/demon, it becomes part of the conjoined creature. A soul by itself if it belonged to a dying human, would flitter off to wherever Buffy went. A demon by itself would just go poof. The vampire's soul, unless removed magically, becomes inextricable from and tainted by its bond with the demon.
Which condemns the poor metaphysical cyborg of a vamp to... whatever afterlife they might end up with, since Angel and Spike are the only souled vampires around. Pavane says Hell, but he's a little biased; Angel says Hell, but so is he.