Hi,
It will require some PHP code editings. You can do it by yourself, just follow my steps:
open this PHP file:
/templates/<yourjoomlatemplatefolder/html/com_virtuemart/user/login.php
and find this line:
<a class="FlexibleResgiterButton" rel="facebox" rev="iframe|480|560" href="<?php echo JRoute::_('index.php?option=com_virtuemart&view=user&task=editaddresscart&addrtype=BT&tmpl=component&flexible=register',$this->useXHTML,$this->useSSL) ?>" title="<?php echo JText::_('COM_VIRTUEMART_REGISTER_AND_CHECKOUT'); ?>"><?php echo JText::_('COM_VIRTUEMART_REGISTER_AND_CHECKOUT'); ?></a>
Then change it with this:
<a class="FlexibleResgiterButton" href="<?php echo JRoute::_('index.php?option=com_virtuemart&view=user&task=editaddresscart&addrtype=BT&flexible=register',$this->useXHTML,$this->useSSL) ?>" title="<?php echo JText::_('COM_VIRTUEMART_REGISTER_AND_CHECKOUT'); ?>"><?php echo JText::_('COM_VIRTUEMART_REGISTER_AND_CHECKOUT'); ?></a>
To do it for "Checkout as Guest" button as well (basically you are going to do the same thing)
Find this line:
<a class="FlexibleGuestButton" rel="facebox" rev="iframe|480|560" href="<?php echo JRoute::_('index.php?option=com_virtuemart&view=user&task=editaddresscart&addrtype=BT&tmpl=component&flexible=guest',$this->useXHTML,$this->useSSL) ?>" title="<?php echo JText::_('COM_VIRTUEMART_CHECKOUT_AS_GUEST'); ?>"><?php echo JText::_('COM_VIRTUEMART_CHECKOUT_AS_GUEST'); ?></a>
change it with this:
<a class="FlexibleGuestButton" href="<?php echo JRoute::_('index.php?option=com_virtuemart&view=user&task=editaddresscart&addrtype=BT&flexible=guest',$this->useXHTML,$this->useSSL) ?>" title="<?php echo JText::_('COM_VIRTUEMART_CHECKOUT_AS_GUEST'); ?>"><?php echo JText::_('COM_VIRTUEMART_CHECKOUT_AS_GUEST'); ?></a>