Is it me, or
Aperture's handling of the vault is a good first step, but it's
still inadequate. Say you have a Library you fill up your drive
with (not recommended). You export enough to burn on DVD (or
DVD-DL) and start to free up space. You use an external to sort of
mirror a larger version of your Library. So, what if you lost your
DVDs and wanted to get stuff from the vault a few years later,
after the vault has grown to over the capacity of your internal
drive? Not good.
Unless I don't understand your example, I"m not sure you fully understand how the vault works. If you have exported projects to DVD and then deleted them from the Aperture library they will ALSO be deleted from the vault the next time you update the vault. The vault is simply an up-to-date copy of your current aperture library. So there is no way for it to "grow" over the capacity of your internal drive.
As to the exported projects on DVD, unless you are using gold "archival" DVDs, good practice is to keep at least 2 copies (recordable DVD's can fail too from surface scratching or from dye deterioration), and periodically (say every 3-5 years) make fresh copies -- and don't lose them.
One other option, if your external is big enough or if you have a second external, is to export your projects to both DVD and the external (separate from Vault) so that you have both a DVD and external HD copy. That way you would have two sources for re-importing projects to your MB based library when you wanted them.