This is located on the second-from-bottom level of Gehennom (immediately above Moloch's Sanctum), and therefore between the 19th and 23rd level of that branch (so between DL 44 and DL 52).
The Vibrating Square level is ineligible to leave bones files.
This is an ordinary Gehennom maze level in most respects. Its floor is too hard to dig through, and there is (initially) no downstair; the only way to proceed is to perform the invocation ritual (reading the Book of the Dead while carrying the lit Candelabrum of Invocation within five turns of having rung the Bell of Opening) standing on the Vibrating Square. Successfully doing so will create the downstair to Moloch's Sanctum on the square itself, and demolish the maze in the vicinity to create a ring of fire traps, a ring of clear space, and a wide moat, in an outwards direction from you.
The Vibrating Square can only be located by the message given when passing over it; it cannot be detected by magic mapping, crystal ball, or any other remote means, and it has no special map symbol. There are some constraints on where it will be randomly placed when the level is created (and once placed, it stays where it is):
It may be on the same square as a boulder or another item.