When does adding a vignette in post-processing improve a photo?

I would argue that the vignette effect cannot be overused. It is only inappropriate or bad use of vignette that can be overused.

If you've ever watched an episode of Top Gear and paid attention, you'd notice that basically every single outdoor shot has had vignette effect applied to it.

It doesn't detract from the show (well, at least not most of the time). In my mind it's proof that done a certain way and applied to certain material, the vignette effect can be used basically all the time and not detract from the material.

Bad use of vignette may involve:

• Adding vignette over the top of overexposure (white highlights) or where you don't have headroom in the highlights. What should be highlights become yucky gray mush.

• Using the same, smooth, oval-shaped vignette no matter what the subject or where in the frame a subject is. If your subject is off-centre, shape your vignette so it's off-centre. Try also making your vignette more harsh-edged.

• Using vignette where it doesn't really enhance the contrast or pop of an image but actually has the opposite effect, de-emphasising interesting details.

• Some vignetting algorithms/filters just seem to be made badly, causing colours to become dull and towards gray instead of more contrasty and towards black. Technically I believe this is because it does not account for the values already being gamma corrected.

It's hard to think of other examples. It may be hard to describe what bad vignette is, but it's obvious when you see it.

Good vignette, on the other hand, boosts contrast, makes an image more edgy, especially the subject within the image. It may even give a framing effect.

Here are some better examples of vignette that I found when researching this answer.