Single Vendor? Do Vendor Assessment; not Vendor RatingANUJ KUMAR SHRIVASTAVA
In the situation when you have a single vendor, there is no point in doing vendor rating, as you have nobody to compete with the vendor.
The best thing you can do here is vendor assessment.
Compare him to the standards you expect him to deliver to you.
For example say his overall Performance is 80%, but On Time factor is just 50%. Then push him to make him understand and improve that.
Try to understand where the fault is and then bring change in policies if necessary.
Meanwhile search for new vendors and develop them. As you develop more, then you can rate them and keep not more than 2 assured vendors for long term business. As you mentioned they are rare or few.