Vignette is not a term I remember ever hearing in school. I learned about them through writer’s groups and conferences. A vignette is a short, well written scene. It does not have a plot, but it does reveal something about the elements in it. It may reveal character, mood or tone. It may have a theme or idea of its own the author wants to convey. It is the description of the scene or character that is important. A blog or diary entry can be considered a vignette.
Wikipedia describes a vignette as “a short impressionistic scene that focuses on one moment or gives a trenchant impression about a character, an idea, or a setting and sometimes an object. This type of scene is more common in recent postmodern theater, where less emphasis is placed on adhering to the conventions of theatrical structure and story development. Vignettes have been particularly influenced by contemporary notions of a scene as shown in film, video and television scripting.”
Try this Vignette writing exercise. http://www.ehow.com/how_2060077_write-vignette.html
Here are some examples: http://www.gabrielerico.com/Main/ClusteringSampleVignettes.htm