We have a simular situation that we are about to test fly a process and see if it works for our 3rd party implementations. 

We decided its really important to we get the essential business processes documented for the business area of impact or for the the solution you plan to replace (As-Is).  Then we will use business use cases, context diagrams and workflow diagrams to model the processes for the customers to verify. After we document all the essental bussiess processes, business rules, non-functional and data requirements, we can use that information to create the functional requirements for the RFPs (To-Be) 

We will then use the  requirements package to verify the 3rd party vendor's solution meets those requirements or to identify the gaps.  The requirements can also become the base for creating the user acceptance test cases.