Hi,
If you disabled the "user registration" via Joomla Users Manager. Then, even though users enter their account details (username, password) via Virtuemart Checkout & Register page; joomla won't let them create account. Because Virtuemart basically rely on Joomla to create the account.
Actually all you need to do is to remove the "Register & Checkout" button from the shopping cart page, am i right?
If so, it is so simple. Just follow these steps:
1- Open this PHP file:
/templates/YOUR_TEMPLATE/html/com_virtuemart/sublayouts/login.php
2- look for this line:
<a class="btn btn-large waves-effect waves-light flexibleModal m-2 d-block" href="<?php echo JRoute::_('index.php?option=com_virtuemart&view=user&task=editaddresscart&addrtype=BT&tmpl=component&flexible=register', true, $this->useSSL) ?>" title="<?php echo JText::_('COM_VIRTUEMART_REGISTER_AND_CHECKOUT'); ?>"><?php echo JText::_('COM_VIRTUEMART_REGISTER_AND_CHECKOUT'); ?><i class="material-icons right">chevron_right</i></a>
3- and remove it.
4- Since you won't let anybody create an account, then you don't need the login form on shopping cart page as well. So you can also remove of the other unnecessary elements within the same login.php file as well (such as; the login form, "member advantages paragraphs" etc.)
5- So, you will only have "Checkout as Guest" button on the shopping cart page; where shoppers click, fill their information and complete the checkout.
Regards.