Yes, so there is a vault there and there is stairs there, they're not one and the same but related. You can traverse the stairs down past the vault which is on this level and not well represented on this map.
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.
This is describing two different things.
You are correct, the whole vault structure is in magical darkness and would extend East and Southeast off the map in a similar way that the rifts, drafts and tunnels of the cave are not on the map, like the place Alice went when she swam through the tunnel under the water.
Consider that now, whether you want to travel down to the next level with the elevator or see where the steps go but after you deal with the new issue.