It seems to me that the langauge definition for COM_VIRTUEMART_MISSING_VALUE_FOR_FIELD is missing in the admin "version" of the language files. It's a setting that is used in the back end, Shop configuration (Vendor) (referenced in administrator/com_virtuemart/models/user.php) - it is present in the front-end language file, but probably isn't ever used there.
I've just checked the download of 2.0.18a and it's still not there.
Okay, it's not a big deal, although it is a setting with a printf %s embedded, so I was missing which actual field was at fault!
Mark