I'm not sure how Vista Mail works (it's not called Outlook Express on Vista), but with Outlook Express on Windows XP, the folders.dbx file needs to have a record of and a pointer to all the other registered mail folders' DBX files. This means that if you backup the DBX file for an Outlook Express folder and then delete the folder from within Outlook Express, the folders.dbx file will no longer have a record of it, and you can't easily restore it or read its contents.
I'd have to play around with this to confirm, and there may be a way to import DBX files, but this is something you should be aware of before you "back up" your e-mail and then delete the only working copy. This is a case where the restore functionality of a backup process really needs to be checked and verified.