This part baffles me as well. The Cessna was at 1200 feet indicated to begin with. AFAIK, Mode C has an error of +- 200 feet. That doesn't sound like adequate

This part baffles me as well. The Cessna was at 1200 feet indicated to begin with. AFAIK, Mode C has an error of +- 200 feet. That doesn't sound like adequate vertical separation to me. Why not ask for a 500 foot climb instead of giving a vector?