It seems like there is no way of displaying a message that the user has not selected a payment or shipment option and wants to continue. It just refreshes the page but there is no indication of what you're doing wrong. I found the code that returns the user to the same page in '../components/com_virtuemart/helpers/cart.php' starting on line 818 but when I replace the code $redirectMsg = ''; with something like $redirectMsg = 'This is a test'; it always shows 'This is a test' even if you are on the page for the first time.
Hello,
COM_VIRTUEMART_CART_NO_PAYMENT_METHOD_PUBLIC, COM_VIRTUEMART_CART_NO_PAYMENT_SELECTED, COM_VIRTUEMART_CART_NO_SHIPPING_METHOD, COM_VIRTUEMART_CART_NO_SHIPPING_METHOD_PUBLIC
Isn't enough?
Regards