Sorry for this as I had a simple tutorial on how to do this from a few years ago and the site has vhange and I lost it. I looked at a few other tutes on this but all have creating a vignette with a black background. If I can avoid using the curves to do this and make it as simple as possible would be best. The image I need to do this to is artwork placed on a white background, instead of having the hard edges that separate the bg and image it would be nicer to have a vignette effect so a soft white background surrounds the image.
I'm in CS6
Put the image image layer above a white background layer. Add a vector mask to the image layer, containing an ellipse or rectangle or any path you like, then adjust feather value in the mask's properties.
Here's an example except it doesn't have a white layer in background: http://forums.adobe.com/message/4551420#4551420