The way I would like implement it would be as follows:
1) When a vendor signs on with you provide them with the sign up on your iDevAffiliate system. They sign up and are effectively a part of your affiliate program and they have a URL that tracks their affiliate sales. For example it might look like the following:
PHP Code:
http://www.captivereefs.com/14338.html
2) When you vendors are promoting your site they would give out the above URL
3) If an end user used that URL to visit your site we would need to add a hook in the registration process to look for the affiliate information. If found and the user completed registration the vendor would get a credit to his account.
After the vendor hit a certain limit in his account you could either pay him directly via Paypal using the functions already built into iDevAffiliate or you could just transfer that into a spreadsheet and use it for a discount against their month advertising.
iDevAffiliate has a workflow that notifies affiliates when a new affiliate sale has been made. If they were interested in tracking these there is an admin interface in the product that allows them to monitor everything.
With the exception of building a plugin for the registration system I think this is pretty straightforward and a really interesting idea that could be great for driving traffic to your site and rewarding vendors for doing so!
Mark