As a Clickbank affiliate, you can append the tid variable to your affiliate link to identify the sources of your sales. But as a vendor, Clickbank doesn't offer any such tracking option! Prehistoric alright.
I've contacted clickbank support and they tell me you can only use the vtid variable on a payment link and this doesn't show on the order form.
How do/can you identify the source of your sales as a vendor on Clickbank?