Hi Matt. You raise an interesting point. What you say makes sense to me.
My general response is: external drives are cheap. Buy what you need and don't spend any time trying to "save" disk space. IME, the administrative overhead is far more costly than the upfront equipment costs.
Here are some specific answers (as I understand how Apeture uses Vaults).
There is no way to exclude Previews from Vaults. Each Vault includes all extant Previews.
Each Vault is a differential back-up. Updating the Vault should update only those files that have been added, changed, or deleted. Unchanged files -- including Previews -- are not backed up anew each time you update a Vault. The amount of time it takes to update a Vault will vary based on the number of changes made, and should have nothing to do with the size of the Library or the number of files in it.
You can test your hypothesis. Delete all your Previews, and create a new Vault. Let us know what you find out.
Suggestions for enhancements should be made directly to Apple via "Aperture→Provide Aperture Feedback". The forum is -- officially -- strictly for user-to-user interaction.