1. Use 'Burn tool' and manually burn-in the edge of your image. This is the easiest method, but the vignet may not be distributed evenly, depend on your artistic hand.
2. - Create a new layer.
- Use 'eliptical marquee tool' on that layer to create an elliptical selection, then inverse the selection.
- Paint it with black color using 'Paint bucket tool'
- Deselect, so that the selection marquee disappear.
- Use blur filter to this layer, choose either one: Gaussian blur or Radial blur. Adjust the blur to your taste.
- Adjust layer blending option opacity, this will determine the amount of vignetting effect.
- Flatten the image.