Many people with type 2 diabetes cannot maintain target blood glucose levels when taking a single oral drug. This is a 28-week extension to a study to assess the safety and effectiveness of vildagliptin, an unapproved drug, in lowering overall blood glucose levels when added to metformin in people with type 2 diabetes who are not at target blood glucose levels on metformin alone. The purpose of the extension study is to gather data on the long-term safety and effectiveness of vildagliptin in people with type 2 diabetes.