A vignette is a brief scene from a movie or play; a short descriptive piece of literary writing; a painting, drawing, or photograph that has no border but is gradually faded into its background at the edges; or a small decorative design printed at the beginning or end of a book or chapter of a book, or in the margin of a page.
In the situation I am sure you are referring to a clinical vignette is simple a short descriptive piece of writing describing a clinical scene - ie, a 65 yo male patient comes into your ER complaining of acute chest pain radiating to the left arm and shortness of breath...blah blah blah
These are being used increasingly to teach medicine, in PBL as well as on the USMLEs.
Hope this helps.