My old faithful American College Dictionary
published in 1951 has a sweet denotation:
" a short literary composition characterized by compactness, subtlety and delicacy."
The OED definition of "vignette" concentrates on pictorial art and technique, from which, apparently ( because of the etymology given) the literary definition derives.
Its essence is a picture or decoration unenclosed by a border and whose edges shade off into the surrounding paper.
For me, a literary vignette has a certain completeness about it: a single diary entry containing a pithy observation; a moral principle expressed with wit; a "no ideas but in things" sort of imagistic portrayal which is memorable in some way; a snapshot in words; a portrait whose after image is recallable; a quick but effective sketch of a landscape, person or creature-- possibly an atmospheric or meteorological moment that seems quickly and completely done, like a bravura watercolor-- small in scale but possessing a satisfying completeness.
Just a few unpolished, tentative ideas here . . .