what file(s) do I need to ensure are backed up in order to protect my library?
You need a backup of the complete Aperture library package "Aperture Library. aplibrary", also of any original images that are referenced.
Have you checked, if your online backup utility works well with MacOS X and can restore aliases and links correctly? For example, putting an Aperture library into a dropbox folder and syncing it there will break it for sure.
I'd try to create a small test library, back it up, also backup increamental changes, and try to restore from the backup. Does it work well?
If you cannot confirm that the on-line back-up works reliably, you can always just make a second vault on a second portable drive, and keep one of the two off-site at all times (never have your three copies of your Library* in the same location).
Slightly less secure, but still very good, is to use Time Machine as your first back-up, and your Vault as your second. Except when up-dating your Vault, store it off-site.
*working copy, Vault1 on Drive1, Vault2 on Drive2.
Another option is multiple Time Machine backups on separate external drives. You can rotate the drives offsite periodically. Or you could buy a fire-proof safe and rotate the backups between the safe and your Mac. Here's a Macworld article on keeping multiple Time Machine backups.
But the online backup is obviously more convenient and more likely to be up to date.
Oddly enough ...
Thomas Emmerich wrote:
Another option is multiple Time Machine backups on separate external drives. You can rotate the drives offsite periodically. Or you could buy a fire-proof safe and rotate the backups between the safe and your Mac.
this review of a fire-proof hard drive just came across my desk.