Hi:
I have updated VM 1.1.8 to 1.1.9 Runs fine but I have followed these steps:
1. I have download patch (updated) from here:
http://dev.virtuemart.net/issues/362
2. Before to install the patch I have created a backup of my site
3. Install patch from VM backend
4. After patched. Replace this file (from backup) because does not work the buttons of quantity:
/public_html/components/com_virtuemart/themes/default/templates/product_details/includes/quantity_box_general.tpl.php
By now runs fine.