Hi,
Open this PHP file:
/templates/YOUR_TEMPLATE/html/com_virtuemart/sublayouts/prices.php
and find this line:
echo '<span class="FlexiblePercentage"> '.$discountPercentage.'% OFF </span>';
and edit the text of
% OFF to whatever you want. Just don't touch this expression:
'.$discountPercentage.' (pay attention to dots and the single quotes)