This may be a solution. I tested it a little and it worked for me, but you should test it some more to see if it works in your situation. Make a copy of, then edit this file:
components/com_virtuemart/themes/default/templates/common/price.tpl.php
Then, just prior to the line:
echo $price_table;
insert the following lines so it looks like this:
// Change the last row of the quantity pricing from, for example, "10 - 99" to "10+"
$price_table = strrev($price_table);
$price_table = preg_replace( "|>dt<>dt/<\d* - (\d*>dt<.*)|", '>dt<>dt/<+$1', $price_table );
$price_table = strrev($price_table);
//
echo $price_table;
Be sure to put that change to the database table jos_vm_table that you made above back to its original value. Let me know if this template change solves your issue.