A vignette was originally just an ornament with a intertwined vine tendril motif. In books the vignette started out as a border of twisted vines, and, shedding its vine motif, came to mean a repeated illustrative element placed at the beginning or end of a chapter. One most commonly comes upon them at the end of chapters in older books where they often seem to play the role of filling all that empty space which many printers seemed then to abhor.

Building on that meaning a vignette can effectively mean any illustration without a frame. For example this wood engraving by Thomas Bewick:

Ralph Waldo Emerson

After the invention of photography it took on the additional meaning of a design in which the central element (often a portrait) was highlighted by removing the background. In the days of photoengraving this was a highly skilled process, involving an air-brush, a paint brush and white ink/paint. You can see this being done towards the end of the first video at Engraving a halftone block. (Even more incredibly in this video you can also see the artist/artisan creating the type by hand using only his paint brush.)

Vignette has now evolved to mean also that effect created by a camera lens whereby the center of the image tends to be brighter than the corners and edges. Your computer’s photo software probably gives you the ability to adjust this if you need to.

Metaphorically the vignette’s meaning was extended in the late 19th century to mean a brief, tightly-focussed written portrait. This meaning has spread out to mean just a sketch in words.