One of the enhancements in Aperture 3.3 is that the Vignette adjustment can now can go negative, effectively devignetting an image. However there’s an important difference between the pre-existing Devignette adjustment and the new negative Vignette ability.
Devignette is applied to the image pre-crop. The purpose of devignette is to remove a shadow (vignette) around the outer edges of a photo caused by either a lens shade or even the lens barrel itself on some extremely wide angle lenses. While we will often add a vignette to our photos as an effect, if you need to crop your image, then the natural vignette may not be your friend.
In the photo above, you can clearly see a natural vignette appearing on the image. The only adjustment made thus far is curves, which has actually amplified the shadow. In the screenshot below, the Devignette adjustment has been applied, and turned up quite high (too high, really, but I wanted it to be clear here).
A negative vignette, on the other hand, is applied after the crop. So if you crop the image in any way, negative vignette won’t accurately remove that shadow. For example, if you cropped into the corner of your original image and tried to use this tool to remove that existing shadow, you could end up with a very odd vignette indeed.
So negative vignette is more useful as an effect than a correction, perhaps to make a dreamy look to your photos or to otherwise brighten the entire outer edge. When playing with it, be sure to experiment with both Gamma and Exposure settings, as the results are quite different.