You could try creating an ellipse and move it where you want. if you cannot see the image under to position the ellipse, then simply set a line around it and clear the fill. Now you can either set the fill again and put it to the back, select both objects and apply clipview. Or you can leave it unfilled and in front and intersect shapes. If you want the edges soft like an vignette you could take the image created earlier and do an elliptical transparency. Either way you do not need to reapply the fill, simply having the ellipse as a line should cut out what you want