Artists have been darkening the outer perimeter of their paintings for thousands of years. A vignette as made by someone like Rembrandt is a nuanced and complex thing composed of depth plane and picture plane elements. The big problem now is that a dumb cousin of the painter's vignette has evolved that has its roots in photography. Old-style photographic vignettes arise from the optical shortcomings of the camera's lens and is entirely situated on the surface of the picture plane.
A more complex, painters-style vignette is shown in the picture plane and the depth plane, with large parts of it being object driven. Below in descending order: the original image, a photo-style vignette (which is only on the surface of the picture plane) and a complex vignette (which involves not only the surface of the picture plane but also the depth plane).
It is a common feature of the vignette to have one corner left open as in the example below by Fredrick Church.
The psd for this image can be found in the Assets page.
A vignette can be considered to be a complex arrangement of Overlap.