The vault itself is made out of typical metal sheets of various sizes, bolted together. On the inside, there are thousands of Krabby Patties lying around and stacked on top of each other, which indicates SpongeBob spends a lot of time and effort in cooking the patties. In large red writing on the front wall of the vault, it reads "Patty Vault" in the same font that time cards and title cards use. The vault seems much larger on the inside than it does on the outside.
Though a refrigeration mechanism is never seen in the vault, it is assumed the vault is refrigerated, because otherwise, the patties would start to expire and develop mold.
Role in series
"Just One Bite"
The Patty Vault appears when SpongeBob sees Squidward hiding a Krabby Patty behind his back in front of the open vault after visiting the Krusty Krab at 3:00 AM. Afterwards, SpongeBob suspects that Squidward likes Krabby Patties, which Squidward, although denies at first, admits and starts consuming massive quantities of the burgers in the Patty Vault before proceeding to explode.
"Mermaid Man & Barnacle Boy VI: The Motion Picture"