Vignetting is also a stylistic choice that many directors make. This can be done to reference the visual style of older cinema types, especially those used in the silent era. Make sure that you have a clear reason to do this and that you can complete the motif in other visual ways.
If you choose to use vignetting in your digital video project make sure that you only do that in post-production in a non-linear editing system that will facilitate such a video effect. This is going to end up looking much cleaner in its appropriation of vignetting, mainly because you can usually decide exactly how much bright and dark you want.
Make this appropriate to the image you have framed, and if you are planning on using the vignetting video effect in post-production you may want to think about this while in production so your images come out with this in mind.