Uninstall Target Coupon. Then, using FTP or NinjaXplorer or some other file manager, go to administrator/components/virtuemart/class and replace the ps_coupon with the ps_coupon from your VM 1.1.8 package. That gets you back to base code. Find the ps_couponxxx (the xxx represents the release you were on when you installed Target Coupon) and delete. That was the save of the original code. I don't want you to use it, since you had multiple installs and upgrades.
Check your coupon functionality. Create a coupon, save it, make sure you are back at base code.
Do a fresh install of the Target Coupon for the version you are running of VM. Then give the code a shot.