Dear Danconia,
1) Which versions of PHP, Joomla, Virtuemart and the Plugin do you have installed?
2) Have you checked in the Joomla plugin manager that the plugin is properly installed and enabled?
3) When creating the custom field, you need to select "Plug-ins" as "Custom Field Type:" first. As soon as you press save, you cannot change the type of a custom field. So if you first saved the custom field before selection "Plugin-ins", you cannot change the custom field type any more.
4) Are there any warnings or error messages in the web server logs? You might also temporarily enable full debug message in VM ("Configuration" => "Configuration" => "Advanced Settings" => "Enable debugging messages" and "PHP developer mode". This should display PHP messages directly on the html page.
If none of these give any indication, I'm afraid I can't further track down the issue without access to an actual system where the issue appears.... If you can set up an admin account at one of the sites where the issue appears, then I can offer to take a look. Maybe I can spot the issue then.
Please send the credentials to office@open-tools.net
Best regards,
Reinhold