Ohh this problem has been fixed already. Are you sure you have the latest version?
Anyway. The patch is very easy to handle, just follow my suggestion and you will be good to go.
1- See the attached file (default_coupon.zip):
Attachment default_coupon.zip not found
2- Download the file to your computer and UNZIP it.
3- You will find 1 PHP file in it:
default_coupon.php
3- Upload that PHP file to this path in your server:
yourdomain.com/templates/YOUR_TEMPLATE/html/com_virtuemart/cart/default_coupon.php
Then, everything will be good to go.
For those who are having the same exact problem;
This patch is valid for "ALL" of our templates.
Regards.
Support