Here's why:
* Definition: A vestibule is a small entryway, hallway, or room between an outer door and the main part of a building.
* Usage: You'd say "I stepped into the vestibule" or "The vestibule was decorated for the holidays."
While "vestibule" can be used in a figurative sense, like "the vestibule of the brain," it's still functioning as a noun in those cases.