Find:
$html_rate .= $CURRENCY_DISPLAY->getFullValue( $value, 5, $currency ).' ('.$rate.'% '.$VM_LANG->_('PHPSHOP_CART_TAX').')<br />';
}
in \administrator\components\com_virtuemart\classes\ps_checkout.php
and change the number 5 to the desired number of decimal places to change the display of multiple tax rates on the checkout page.
Hope this helps.