There is no path to convert wiki pages to normal threads at this time (in VaultWiki 4). If you don't plan to use VaultWiki anymore, then you should do this in VaultWiki 3, before you migrate.
If you migrate to XenForo from vBulletin, then you will need the full version of VaultWiki 4 for XenForo, since the Lite version doesn't have a migration tool. Once you have VaultWiki installed on XenForo, use the regular VaultWiki importer to migrate data from a VaultWiki 3 installation.