You need to have very good reasons to change exiting supplier/vendor. May be you are aware that the competition is not with the companies but one who has the best supply chain. The
classic old metrics are Cost,quality and Delivery. Now the new metrics are Speed,Response and Flexibility. And these factors do cost more. but new metrics take you to a new and High level performance.
In my view maintaining a good exiting supplier is extremely important unless you have valid reasons