This might be a completely different approach however this could help you in the long run.
Develop a simple mobile application for your vendors who would refer customers to you .

1. Customer visits the kiosk , sales person refers and inputs customer info
2. The application sends out digital business card to customers
3. When customers need your service their contact info is already in your system and the referrer gets commission.
4. Every time a vendor enters customer info on the application it updates the database that you can access through a admin portal.

In a way you can also use customer info to send out any future coupons , ads etc to potential customers.

Go with a very basic application so your investment is low and later improve the application based on the ROI

There are other approaches to this as well however if you are up for a little investment this may turn out to be helpful.