Discworld/Small Gods/WMG

Vorbis will eventually come to believe that he's in an unexpectedly cold Hell, and in doing so will further cement his inability to move on.
I personally doubt this is the case, but it seems perfectly plausible that if he even thinks only one new inkling of a thought each millennium, he'll eventually have thought for a living philosopher's equivalent of years and years and come to the conclusion that he's being punished by his exile to a desert that doesn't even have the burning, punisher form of Om let alone the rewarding Om that he'd come to expect from his afterlife. In his mind, then, nonbelievers would get the cauldron but he, a believer who somehow failed the One God, his god, wuld get an eternity with only the false voice of Om echoing mockingly in his head.

Brutha's reform eventually starves Om again.
Accepting the Discworld definition of belief-fueled deities, "ritual-based" worship does not feed such deities. Brutha's reforms basically consist of following a utilitarian path and naming it after Om; and pretty much ignoring everything Om says. While this is "better" than what Omniasm was before; it has little to do with the type of belief that Pratchett said small gods need. Later "brochure-based" versions of Omniasm and Reverend Oats "good works" version of faith (that Granny Weatherwax pretty much says is the same as her naytheism) don't feed him either. Om's only chance for survival is to change his name and start from scratch.
 * Much like the Duchess from Monstrous Regiment, Brutha will replace Om whether he wants to or not. In the Tiffany Aching books, for example, the "good" Om worshippers refer to Brutha instead.
 * Alternately, Om will survive, because he's adopted a strategy of pure Fridge Brilliance: he's living off his worshipers' uncertainty about what he wants rather than their certainty. If there are dozens of schismatic sects out there, all saying that Om wants different things, then all of his worshipers could be in the same boat as Mightily Oats: unsure of which Om is correct, and worried about whether they're pleasing him or not. In which case, they're still demonstrating a belief in Om through their anxiety -- something that must be sufficient to keep Om alive, if saying "Oh, god, what did I just hit?!" when your wagon squashes a small animal is enough to sustain a minor deity -- just not a belief in any particular characteristic of Om.

Angus is one of the small gods of the desert
He obviously has an objective existence, and what better explanation than that he's a small god who's latched on to St. Ungulant and gained belief in return for companionship. This stronger belief would explain why he's somewhat more powerful than the other small gods, even to the point of being able to manipulate objects.