A vignette works like a mat or frame to visually guide the viewer into the more important center content.
When a dark vignette is added to a photo with light edges and darker core content it has the opposite effect, becoming a distraction that catches the eye and pulls it back out to the edges.
More effective on photos with lighter edges is to vignette lighter in the corners to deliver the same subliminal perceptual message as darker edges on a middle-tone sky or frame edge, "Go look at the more important stuff in the middle and stay there."
Either way if the viewer consciously notices the vignette it becomes a distraction and fails to work as intended. I vignette the edges of most of my shots with tone and blur, but not so much that anyone usually notices
