The following SQL query does nearly exactly what it should but I need to add one more thing to it and I am unsure how?

SELECT viewRequests.Course, 
       viewRequests.Priority, 
       viewRequests.ReqDate, 
       viewRequests.FirstName, 
       viewRequests.LastName, 
       viewRequests.Department, 
       viewRequests.Status, 
       Rates.Price, 
       viewRequests.PriorityID
FROM viewRequests 
       LEFT JOIN Rates ON viewRequests.CourseID = Rates.CourseID 
          AND viewRequests.DeliveryDate <= Rates.ExpiryDate 
          And viewRequests.DeliveryDate >= Rates.EffectiveDate
ORDER BY viewRequests.Course, viewRequests.LastName;

The tables it uses look like this:
viewRequests                  Rates
------------                  -----
ID                            ID
ReqDate                       VendorID
DeliveryDate                  CourseID
Course                        EffectiveDate
CourseID                      ExpiryDate
Priority                      Price
PriorityID
FirstName
LastName
Department
Status

The problem is that there can be two or more sets of rates from different vendors that have the same date range and I want to select for each request the lowest possible price.