$vars['mailbody'] =vmText::sprintf('COM_VIRTUEMART_PRODUCT_LOW_STOCK_EMAIL_BODY',$link, $vars['product_in_stock'] - $vars['product_ordered']);
assuming there is a $vars['product_ordered']
check first
print 'Debug Line '.__LINE__.' pos <pre>'; print_r ($vars['product_ordered']); print "</pre><br />\n"; die();