Item name.
(an unconscious goblin)
Vestibule access is a drop quest from the 'corkscrew room' event. Basically, there's a circular room with 3 levels of ramps around the outside, and a spiral ramp that looks like a corkscrew in the middle. Run up to the top of that corkscrew, and it will spawn a number of waves followed by a named or two. They drop the starter. You can then go down the hall to a previously locked door, re-enter the room and run the event again.
Borxx also drops the vestibule access starter, although this is a sub-optimal way to acquire them.