Vignetting is an effect used to simulate the limitations of some cameras. Therefore, it doesn't make no sense at all to have it in first person, given that you're supposed to be looking from your character's eyes.

However, having it for third person still makes sense, considering that the 3rd person perspective is meant to simulate an actual camera.

You guys already do this with lens flares, they are only visibile in third person (or when using a scope), why not do the same with the vignetting effect?