Did Zul'Aman favor Vilebranch over Revantusk, specifically? From the quest dialogues I recall the Revantusk acting like
THEY were the Zul'jin loyalists, with the Vilebranch simply being particularly depraved barbarians that took over the region's biggest city.
Such a question would compel us to investigate what Zul'jin's foreign policy is. In brief, the Amani spent hundreds of years being beaten back by the Elves and Humans and then joined up with Doomhammer to finally try and destroy the elves for good. Turns out after having a go at getting through the forcefield they can't manage it, so Doomhammer just leaves and goes and gets defeated without accomplishing anything during the Second War. Really not an impressive performance from the Warchief tbh.
Meanwhile the Amani (and I would assume their cadet tribes) are defeated by the High Elves sallying out, Zul'jin gets captured and then escapes, and then seems to retreat back to Zul'Aman. Presumably the Amani et al were weakened by the failed attempt, and then by a subsequent attempt to attack the Sunwell. Then the Scourge attack, mainly aiming at the elves but hittng the Mossflayer too we know.
So the state of affairs is that the Amani and their vassals are weakened by a series of events, and feel it necessary to turn to more extreme methods later on with the whole 'sacrifice our gods' thing.
The Revantusk apparently specialised into diplomacy. I don't really know what that means but presumably they brokered deals or maybe lent out people to make deals or something. They decided, for reasons best known to themselves, that they would retain their alliegance to the Orcs and Horde rather than the Amani. We don't know why that would be, but again we can assume that Chief Torntusk felt it was the advantageous course of action. We also know the Smoulderthorn and Firetree tribes left to go hang with Rend, so again we can assume that them and the Revantusk perhaps were disadvantaged in the Amani heirachy.
Comparably, other tribes like the Witherbark and Vilebranch stay around, acting as the southern bulwark of the Amani lands. The Revantusk are preyed on for sacrifices by the Vilebranch in particular, and have been pushed out of the cities into a pathetic little village by the beach. In this quest therefore I've got them at roughly 2k numbers, wheras the Vilebrach have multiple cities so would be at 50k maybe. The Revantusk regard the cities as theirs, but yknow how did they get pushed out? Were they just really bad at fighting? The Vilebranch are clearly massively more powerful militarily.
It's weird because in game it's one of the more extreme examples of a faction just sitting about while mercs do their work for them. The players do all the work to get the land back, the Revantusk just squat on the beach and moan about their chief getting captured etc.
Again, I'm basing some of this on the Aztec Flower Wars, the idea being that the trolls practice a form of ritual warfare designed to reduce large casualties and to acqurie sacrifices, as such maybe the Revantusk were just trying to get away from this style of warfare etc and wanted to not get kicked about all the time.
So as to your question, it relies on the history of the different polities. I suppose that Zul'jin and the Amani's foreign policy priorities are to acquire allies, retain vassals and to destroy the elves. As such, they probably don't care or are willing to overlook the Vilebrach bullying the Revantusk. I doubt Zul'jin favours the Revantusk, I suspect he was very upset and perceives the Orcs to hvae betrayed him or lured him to attack Quel'thalas, but he's probably cooled down a bit now, and is thinking on more strategic lines
