I made IT! (Note -> VM 2.0.1)
Here is my solution:
Insert the following code under the // vmdebug('cart',$this->cart); (line 24)
1.
$url = JRoute::_('index.php?option=com_virtuemart&view=vendor&layout=tos&virtuemart_vendor_id=' . $this->cart->vendor->virtuemart_vendor_id.'&tmpl=component');
$document = &JFactory::getDocument();
$document->addScriptDeclaration("
jQuery(document).ready(function($) {
$('a.tos').click( function(){
$.facebox({
iframe: '".$url."',
rev: 'iframe|550|550'
});
return false ;
});
});
");
2.
Replace this code <?php echo $this->cart->vendor->vendor_terms_of_service;?>
to <a class="tos" href="<?php echo $url ?>" ><?php echo JText::_('COM_VIRTUEMART_CART_TOS') ?></a>
You are finished

Now the TOS will open popup box like "ask a question". You can make some css style for the link and you get shomething like the attachment

[attachment cleanup by admin]