Aperture 3.x: What Is a Vault?
To ensure that you have backup copies of your photos, you create a vault to hold the backup files. A vault is a container that holds an exact copy of the library. This includes projects, originals, and any versions you’ve created. You can easily create and update a vault to back up the library. It’s a good idea to create multiple vaults on multiple external hard disk drives to safeguard copies of the library.
You can have as many vaults as you deem necessary. Creating more than one vault is useful if you work at different locations; you can always keep one vault on an external drive onsite and another one offsite. All vaults and backup files are tracked by the library so that even if you disconnect the external hard drive that contains a vault, Aperture can access it the next time you reconnect the drive and update the vault.
All the originals and versions for managed images are backed up, as well as all metadata, previews, and adjustment information associated with managed images. The versions, previews, metadata, and adjustment information associated with referenced originals are also backed up in the vault. Referenced originals are not backed up in the vault with the library.