The dilithium vault was a restricted location aboard Constitution-class starships where dilithium was stored. It was guarded by security officers.
In 2269, Bell and Gabler were assigned as relief guards for USS Enterprise's dilithium vault. While on duty, they discovered two of the Klingons assigned to combine the forces of the Enterprise and IKS Klothos looking around the vault, and brought them to the bridge. The Klingons explained their presence there as the result of having gotten lost, but Gabler didn't believe their story. (TAS: "The Time Trap")