Originally the vignette was a technically undesired featured of an image caused by limitations in optics during capture, printing or both. An unintended but desirable side effect was to draw attention into the middle of the image. Modern lenses still create a natural vignette when used wide open but most applications for digital post processing provide tools for intentionally putting in a vignette.
So now it is a matter of taste and art. This is how I saw the reed flower and how I chose to present it.
Photo: Robert Rath, 'Day 386, Tall and True' 1/1600s f/2.8 ISO640 120mm
... more at http://robertrath.com