Lots of generalities incoming:
Think deeper, 'Faces of Evil' type of vile material, not shallow stuff that's more icky, gross, or revolting rather than Evil and seriously dark.
Disturbing in every meaning of the word, not blood and guts squick factor. The Exorcist rather than teen slasher movie from the 80s.
PrCs... not sure I'd go for that myself. Vile acts in and of themselves can be done without needing a special class for them.
Items are probably where you may have more luck. Detailed items with history and backstory, something to seriously inspire with information as black as you can make it. Vile type artifacts would find a market I suspect.
Monsters... fiends in and of themselves should qualify for anything 'vile' in the book that you can think of, so individual monsters with fleshed out backstories will probably suffice.
Personally I like NPCs that are truly and irredeamably vile, usually, about 90% of the time these will be fiends. Just to give examples of material and ideas that I'd consider 'vile', and a bit more than the normal black and white evil you find presented in typical DnD games, maybe something just completely over the top like...
Excerpt from The Flesh Sculptor said:
Blotches of sudden white light awoke Selkar from the darkness. Glowing, glistening spots of color danced across his field of vision as his eyes involuntarily jerked and pulsed in their sockets with both pain, and the beating of an alien heart and the rush of alien blood into his brain and through the hemorrhaging vessels across his retinas.
“Welcome back mortal of mine.” The Baernaloth’s voice whispered into his ear. “The strings of the puppet are hung from the playwright a gallows upon which to dangle and now dance.”
While the Baernaloth rambled, perched precipitously on the edge between brilliant lucidity and outright psychosis, Selkar was moving. Or the room was moving. He wasn’t entirely sure. But no… the perspective was all wrong, skewed, as if he had been shrunk down to a fraction of his prior height. He was following the Baernaloth, looking up at it, his hands pressed to his cheeks to tilt his head at the correct angle.
Selkar’s hands did not have claws. They never had. But the hands upon his cheeks did.
“Observant at last, little rat to my pied pipe.” The Baernaloth said with a chuckle, turning its head to look down at the summoner. “Such skill would have served you much better before you attempted to summon me.”
The fiend gestured with a finger and Selkar’s field of view rotated, his body turning and swiveling to follow the directions of its puppet master. What he saw brought spots to his eyes and a soundless scream from his lips.
Where the Flesh Sculptor pointed, sprawled in a bloody mess on the floor, was Selkar’s headless body. A foot or two away was the severed head of one of his apprentices’ quasit familiar. The alien hands on his cheek pulled his face down to look at the body that supported it.
Slick, mottled blue flesh, webbed, frog-like feet, a gently twitching tail, and missing or ambiguous genitalia. The Baernaloth had connected his head to the still living body of a quasit, the same body that was following it in a compulsive, ecstatic trance.
And if you want to be more subtle, I like them especially when they can drag PCs into moral quandries. Something like...
excerpt from The Blind Clockmaker said:
The Gloom Father smiled and laughed, “So I did say that, promised you an answer… as for the bargain they and I made, some things happen because they must. Accept that mortal and live your life under all those moral pretenses you hold onto. Nothing comes without sacrifice. For anything to happen, anything great to happen, there must be two things: blood and terror.” The Clockmaker paused and turned its sightless eyes towards the elder Ratatosk. “We are well acquainted with both…”
The Baern stepped closer to the elder ratatosk as the children whimpered in abject horror, clutching onto Clueless, Tristol and Aiden. “Uncover their eyes. Make them watch this.”
At least that's what I think of when I think of 'vile' material. I go for much for the fluff side of things than looking for crunchy stuff, but the market may differ dramatically from what I'm looking for. I can't really say.
Organizations, cults, richly detailed NPCs/monsters, and artifacts that carry around significant baggage, those are all ripe for work I think.