Hi Fred, unfortunately using "only" guest checkout feature is not supported by Virtuemart. There is no parameter in the Virtuemart Dashboard about this to activate it easily;
But you can follow these steps:
1- Disable user registration via Joomla Settings (so no one will be able to register -- See here:
docs.joomla.org/J3.x:Disabling_user_registration/en)
2- Hide the "Register" and "Login" elements from Virtuemart shopping cart page.
These edits will let you have "guest only" checkout system in the shopping cart page.
FYI: To hide the "register" and "login" elements from Virtuemart shopping cart page; you need custom coding in some of the template override php files. Unfortunately we don't provide step-by step guidance for custom coding modifications. But you may purchase premium support service hours via
www.flexiblewebdesign.com/premium-support and let us take care this customization for you directly on your website though.
Regards.