I am sorry for taking so long to respond. I am now semi-retired and missed you post.
We have had several member companies who have included critical vendors in their gainsharing systems. One company included the vendor who handled the electrical systems in an Aluminum Anodizing company, the other was a custodial service in an automotive parts company.
The key in both cases was the vendor was part of the larger gainsharing system. I think you are asking for trouble if you try to create a different reward system for each vendor. Also these vendors were critical to the success of the company and were visible to the employees every day--they were very much a part of the company.
They submitted suggestions and used the company continuous improvement (suggestion) systems.