Hi,
Once you checked the "Only registered users can checkout" option in VM Configuration, then the "Checkout as Guest" button in the shopping cart should be gone.
When the "Checkout as Guest" button is clicked from shopping cart, there is no field for "username and password". I don't understand why you are saying. with "Checkout as Guest" option, buyer can finish the checkout without logged in. So the username and the password is not the case.
The error messages appeared by your joomla template.
Virtuemart and Joomla shares the same user manager system. So while people are trying to create an account through Virtuemart, they actually create an account in Joomla. So if there is an error prior to register, then the error message is shown by joomla. Not by Virtuemart, and of course not by our VM Template.
Make sure your joomla template layout PHP page (/templates/<yourjoomlatemplate>/index.php) has this line somewhere:
<jdoc:include type="message" />
Where ever this line is located in your joomla template, the system info/warning message regarding to anything will be printed in this section.
This line usually stays above this:
<jdoc:include type="component" />
(which where your joomla template print the "joomla articles". But your joomla temlate might use a different structure to show these. I can't be sure.
Regards.
Forum Support.