I've murdered you image Chistina working on each layer as they are created.
Duplicate layer mode overlay, call it contrast, desaturate and invert. Adjust opacity to enhance detail as needed. This one is 50% to just bring it up slightly. It would be hard to do this evenly with a brush.
Add a new layer calling it vignette, black, mode soft light. Add a layer mask, white full opacity and select while working on the mask. Select a larger area than the parts that shouldn't be be changed. I used a rectangular select with rounded corners and some fuzz. Fill that with black, full transparency, de select and then use a rather huge gaussian blur to smear it out more. Adding the black will bring the underlying image back to how it was. This is a conventional darkening vignette. I then replaced the layer with it's mask and inverted it and change the layer mode to screen. That gives a white vignette. Looks like this could be tinted if needed or even the whiteness changed.
All GIMP but the point is that each step is made with a fresh layer and the effects can be seen by leaving all visible and just selecting a layer and changing each as desired. Not knowing the layer mode doesn't really matter. Try all until you find the one you want. Vignetting is done like this so that it can have any shape even a free hand selection so there should be an Adobe tutorial about. That will probably just cover the usual vignette but invert and changing it's layer mode should do the same thing.
Make a mistake - just delete the layer and do it again.
Just for fun I added a white bezel edge and a blue black frame plus gradient that bled into the bezel. Also increased contrast of the whole image a little as the above did seem to loose a little or maybe I thought it looked a bit flat.
When adding a frame I selected new layer from visible and added it to that so that it would be possible to just delete that and do the same thing again if needed. Some people always duplicate the original image so that any changes can be removed if needed.
John
-