If you take a look at the size of your Autosave Vault folder it's likely huge. Mine is 4GBs right now. Every once in a while, when I think about it, I archive it off and save some space.
Why archive it at all you ask? It's nice to have saves along the editing process so you can go back and pull up any changes from long-ago or specific tests or whatever bizarre things clients tend to ask for years later. Another reason to archive them (meaning compress them, also) is since they're essentially just .xml files they compress nicely.
I archive my Autosaves in all those .fcp folders which requires a tiny bit of setup because rather than archive all those folders into one huge archive we're going to batch compress each folder into it's own archive. This way they're still named and individually archived so it's fast to get to them and transport them if need be.
It's not too hard to just right click on the whole Autosave Vault folder and compress it into a .zip archive. On my drive here my Autosave Vault folder is about 4GBs, compressed though it's 291MBs. That's a huge amount of space saved. But it's also messy if you want to just get at one particular project's autosave (.fcp) folder.
Here's what I like to do instead:
1. While in the Finder hit COMMAND + OPTION + G and in the resultant dialog box type "/system/library/coreservices" (capitalization doesn't seem to matter).
2. In the new window that opened up locate "Archive Utility" (it may be called "BOM Utility" or "BOMArchiveHelper" or something like that depending on the version of OS X you're using). BOM stands for Bill of Materials, if you're wondering.
3. Optional: Drag the app (it's okay, really) to your Dock or Sidebar or make an alias of it where you find it handy.
4. Launch it.
5. Hit COMMAND + , (comma) to open its preferences.
6. Look for something like "Use archive format" and set it to "Zip archive". Close the prefs. (Close the Core Services window too, if you'd like; we're done with it.)
7. Now, highlight and drag all the Autosave folders inside your "Autosave Vault" (not the Autosave Vault folder itself) onto the Archive Utility icon and watch the magic. It may take a while depending on how many projects you have and how many files are in those folders.
8. Once it's done you'll have a messy mix of folders and compressed folders. Simply set the window to List view (Command + 2) and then tap the Kind sorting column. Highlight all the "Folder"s and trash 'em with Command + Delete then Empty trash with Command + Option + Delete.
9. Quick Archive Utility.
10. Place those archives wherever you want them. They're tiny and portable and archive nicely to Dropbox or Box.com.
11. There is no step 11 :)
Tell DropBox if you use it to backup your autosave vault and you'll never lose another save. If you use Box.com you'll need to tell FCP to save into Box.com's folder since as of this writing Box.com doesn't understand aliases or even symlinks.