I have all my pictures on an external drive (referenced media), and I do clone backup from my internal ssd, and from my external drive. Are there any reasons to have additional backup as a vault?
MacBook Pro with Retina display, OS X Mavericks (10.9.1)
Reason for vaults? They are incremental backups of an Aperture library and save a little space by excluding redundant information, like the Photo Stream images. Vaults do not backup your referenced images. You can achieve the same by cloning your Aperture library. If you have a working backup of your Aperture library and your referenced images, you do not need vaults. I'd try to have at least two backups of my phpto libraries created by different applications on different drives.
The Aperture library includes all your adjustment metadata, keywords, etc. A vault will preserve this, even if you're using referenced images. I'd recommend it.
Vault backup and referenced media
