Vignettes: elements of a composition that frame the subject, usually in a oval-ish shape. Effective vignettes should point your eye to the most important part of the pic. If your background doesn't have a natural vignette, one can throw in a shadow vignette in photoshop, like that to the left. The edges fade to dark or light (see example of a light fade in a previous post, the sunshiney pics).
There are different levels of shadow vignettes: the fade can be sudden or gradual, the edge may be black or just slightly darker, the fade might start from the middle or closer to the edge of the photo, the fade may be oval in shape or irregular. Vignettes should be tailored to the photo. What make me cringe is seeing vignettes that have been applied willy-nilly, regardless of composition and subject. Like any edit, whether it's B&W conversion, color enhancement, sharpening or grainifying - ideally, these things should be selectively applied to contribute to the meaning of the picture. In terms of portraits, I think shadow vignettes are good for private/thoughtful moments, relatively closely cropped pics, in B&W.
Then there are compositional vignettes, created at the time of photo-taking. One searches out environmental features (whether it be architectural, natural, lighting etc) that frame the subject. More obvious vignettes is to place the subject under an archway or in a doorway. To the right, we have a natural vignette, a frame of tree trunks and branches. A generous foreground of grass balances out the heftiness of the trees. The jumble of branches to the left is a little distracting - so not the perfect set up, but I think one's eye will linger longest on the couple.