This occurs because in the most recent edit (after the import) someone has typed an ATTACH BB-Code that points to an attachment which is attached to a different post. Since the ATTACH was added after the import, it is not automatically imported as a new wiki attachment.
The ATTACH BB-Code rendering as a link is the expected behavior for this case in both vBulletin and XenForo, even if VaultWiki is not used. When the specified attachment is not on the same post that contains ATTACH, the code renders as a link.