Start by filling your background canvas with a light grey color. In a new layer, fill with a darker grey. Then, create a feathered rectangular marquee with about 1 inch from the edge. Hit delete to make a hole in the darker layer, and you'll see a lighting effect. Merge this with the background layer to create a flat vignette. Use Filter > Distort > Lens Correction, and you can intensify your vignette. Scott makes copies of the layer and manipulates them to create a light box effect.