Modify /components/com_virtuemart/router.php, line 130-135 from:
case 'productdetails';
$virtuemart_product_id = false;
if (isset($jmenu['virtuemart_product_id'][ $query['virtuemart_product_id'] ] ) ) {
$query['Itemid'] = $jmenu['virtuemart_product_id'][$query['virtuemart_product_id']];
} else {
to
case 'productdetails';
$virtuemart_product_id = false;
if (isset($jmenu['virtuemart_product_id'][ $query['virtuemart_product_id'] ] ) ) {
$query['Itemid'] = $jmenu['virtuemart_product_id'][$query['virtuemart_product_id']];
unset($query['virtuemart_product_id']);
unset($query['virtuemart_category_id']);
} else {
Simply added:
unset($query['virtuemart_product_id']);
unset($query['virtuemart_category_id']);