sketch vs vignette

sketch and vignette both are nouns.

Word	Noun	Adjective	Verb	Adverb
sketch	Yes	No	Yes	No
vignette	Yes	No	No	No

As nouns, vignette is a hyponym of sketch; that is, vignette is a word with a more specific, narrower meaning than sketch:

sketch: preliminary drawing for later elaboration
vignette: a small illustrative sketch (as sometimes placed at the beginning of chapters in books)

Other hyponyms of sketch include design, draft, rough drawing.

As nouns, sketch and vignette are synonyms defined as:

sketch (noun)	vignette (noun)
preliminary drawing for later elaboration	a small illustrative sketch (as sometimes placed at the beginning of chapters in books)
short descriptive summary (of events)	a photograph whose edges shade off gradually
a humorous or satirical drawing published in a newspaper or magazine	a brief literary description
a brief literary description

sketch (verb)	vignette (verb)
describe roughly or briefly or give the main points or summary of
make a sketch of