The way I do the vignetting of my images is based on attracting the attention of the viewer to the subject of my images. Often my subject is not in the centre and I'm very happy I don't have to use those preshaped vignettes on my camera anymore since I started with digital photography.

Usually I sopy a layer, darken the edges and other spots that might distract with the burning tool or paint them over with a rather dark grey. I make it much darker than needed and afterwards I change the opacity to the desired amount. This way you are not limited to the use of ovals, rectangles etc.

Hope this works for you too.