Marc,
Ok- now I am not quite sure where to go next.
The Coupons tab / menu item is not showing at all in the dashboard?
I tried to create a vendor account to see, but you have a lock/password protection on your site, so I could not login as my vendor.
Have you deactivated all except WC, WC Vendors and WC Vendors Pro? Just to rule out any plugin conflicts… and also , since I know you have done a lot of modifications to your templates and added a lot of functions to your functions.oho— could you try quickly switching over to the standard non-modified storefront parent theme to see if anything you have done with the templates or the functions.php has perhaps caused this to occur?
this will help with trying to isolate this issue..