Ik think the AP describes it pretty well tbh. Fey subsist on grubs, leftover humanoïd servants turned to cannibalism, other creatures are kept in stasis etc.
The lowest part of the dungeon had a working farm to supply the Vaults.
The mitflits are likely foraging the swamp. The upper and middling denizens include many undead, outsiders, and aberations that can presumably subsist for long periods (or indefinitely) between meals. Also recall there are tunnels connecting to the Darklands here, so inter-faction predation supported by wildlife and wanderers that come in from these tunnels should suffice till the farms and hunting grounds are accessible in the lower third.
Hey folks. Just wondering for those more familiar to the module -- is it ever explained how the creatures survive, and what they eat? To me, for a dungeon to be believable, they have to be able to sustain themselves via an abundant food source that all denizens can access in some way or another.
So, what to the dungeon denizens of the Abomination Vaults eat?
The morlocks are probably foraging the swamp at night. Their leader offers the PCs a quest to kill the drake, who makes it difficult for them to move around; but their part of the ruins is described as being full of waterfowl bones and big beetle shells. So they must be heading outside for food, dodging the drake as best they can.