Wait what?
I thought the stairs were the vault?
Where is the room and treasure Ashley talked about? I thought it was where the ladders led into the darkness?
On the furthest southwestern wall next to the drift, there is a very old metallic structure, made of that same dwarven metal, built to survive anything—well, almost anything. There are remnants of what was once a very sturdy metal seal here; it looks like it was recently opened, and by the looks of it, with literal barrels of explosives, enough to make even Alice blush. The metal hinges are partially melted from the blast. A deep black tunnel remains, too black to be natural. To the southeast of that is a wooden platform with stairs leading down. The stone that once separated the drift from the other side has recently fallen away, revealing a fresh rock surface, likely from the blast.
Ashley refreshes her Truesight and she describes what she sees in the magical darkness.
"Dust, ash, walls blackened by fire, and heaps of debris beneath a sagging ceiling—yeah, this room was damaged by a destructive blast. I can see remains of furnishings—tables, chairs, bookshelves, and beds—are charred; this was some kind of living quarters. Wait a minute..." Just on the edge of darkness, a scorched iron chest stands proud and blasted open. Glowstone light piercing the darkness shows glints of gold and the tell-tale hue of Mythril--a lot of it. A rope, ladder, or other means could be used to reach the chest.
Oh wait. So the whole vault structure isn't on the map?