The general rule for backing anything that you care about is this:
- There must always exist at least three copies of the data, each copy on it's on physical media.
- data must be stored at a minimum of two geographic locations
With just one Time machine disk all have is just one copy at one location. You are still "short" two physical copies and one geographic location. If you don't follow the above rules you will loose the data, it's only a matter of when.
You can get all the way there with just TM but you'd have to have multiple TM disks and rotate them keeping at least one off site at all times. TM really is not meant to be used this way. It is best to have just one TM disk, as big as you can get and leave it connected.
A "vault" is specific to Aperture and allows you to very quickly make a complete and consistent copy of the library at a point in time you choose. If you already use TM them get a couple more external drives and make two vaults. keep one at (say) your office and rotate it back with another one you keep in a fire safe at home Periodically connect a vault, sync it them take it to the office and bring the old one home to be re-sync'd. Also let TM run. This is exactly what I do. I just bought a 1Tb disk for $150. Disks are so cheap there is no reason not have a good backup system
One other very important difference between TM and Vault. TM copies files one at a time so if Aperture were updating one file and then another. Say saving an edit then updating the database. There is a small chance that the the two files will not match because TM's backup is NOT a "point in time". Each file is backed up at a slightly different time. "Vault" is in fact a clean selft consistence snapshot of the Library. This may be a minor point because eventually you will close Aperture but TM will continue to run every hour. But I'd not trust TM backups made while I was using Aperture