The vault is only seen in the episode Just One Bite. It is shown when SpongeBob sees Squidward eating many Krabby Patties in the vault, after visiting the Krusty Krab at 3:00 AM. Even though Squidward claims he hates Krabby Patties in other episodes, he becomes addicted to them in this episode.
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 Krabby Patties. In large red writing on the front wall of the vault, it says "Patty Vault," in the same font that timecards and titlecards use. The vault seems much larger on the inside than it does on the outside.