Not sure why Virtuemart isn't detecting your menu item for itself correctly. It basically looks at your menus and selects the Virtuemart page it thinks is 'best' for the registration page, but obviously not working.
Either way, if you can make the following minor modification to the JFBCLogin module, it should get you going. In the /modules/mod_jfbclogin/mod_jfbclogin.php file, around line 75, you should see:
$registerLink = $sess->url( SECUREURL.'index.php?option=com_virtuemart&page=shop.registration' );
Change that to (adding the Itemid=2 to the end):
$registerLink = $sess->url(SECUREURL.'index.php?option=com_virtuemart&page=shop.registration&Itemid=2');
Hope that helps, and let us know how it goes!
Alex