How about you do the same edit except do a white vignette instead of a black one? If you're in Photoshop, create a white layer behind the image. Then choose a circular selection, feather 150-250 pixels, and draw a circle around her. Select inverse, then delete the selection. I think it'll have a two-fold benefits--first, the light colored vignette will compliment the high-key portrait instead of detract from it, and second, you can lighten her hair ifthe hair isplaced in the gradient between thesubject and vignette, therefore making it less distracting.