A vignette as it refers to literature is a short, artistic piece, usually spanning no more than one scene or event.
Online, vignettes are sometimes called ficlets or one-shots, and usually involve a character, rather than an action-driven plot. They can be fluff, angst, lemons, or any other type of fic.
Wherever you find them, good vignettes convey a strong dose of evocative prose in a small space, and they can be a joy to read.