Grab the elliptical marquee tool and make a selection of the part of the photo you want to use. Right click within the selection and select feather from the menu. A new dialog box opens and you need to set the feather radius to around 20 and hit OK.
Next you are going to choose select>inverse and what you will see is marching ants around your marquee selection AND around the edge of the entire image. Hit the delete key and everything except your marquee selection will vanish like magic.
Make a new layer and move it to below your feathered selection (the pic above). Using the eyedropper tool pick up a color from within the photo and left click once. That will make your color selection active in the foreground chip. Fill the selected blank layer with your selected color.
At this point I thought it would be really cool to vignette my lovely couple with some flowers so I found a photo of some daisies and opened it up!
At this point all you have to do is drag the flowers onto the color filled vignette layer and you are done!!
Here is the final image.