Yes, its how the username field is removed from the registration page. Sadly Virtuemart doesnt support template overrides, otherwise template overrides would have been used. Can you tell me which version of EmailAsUsername you are using? The latest version should only copy in the modified ps_userfield.php when enabled.
If you want to change ps_userfield.php then change EmailAsUsername's copy in /plugins/system/lunarplugin/template_overrides/com_virtuemart/<vm_version_number>/classes/ps_userfield.php
Then disable the plugin, and enable it again.
You can get support at
www.lunarhotel.co.uk/help/forum if you need further assistance
Dylan