A vignette is just a simple scene or brief image. A diorama usually entails a bit more and involves a larger picture. In IPMS competitions it would be anything with more than 3 figures and or a combination of model(s) and a believable setting that tells a story, or sets a mood.
i.e. a Diorama would be a Sherman tank and some ground infantry breaking through a hedgerow. A vignette would be a couple of those soldiers and a portion of that hedgerow as maybe a back drop for them. Diorama would be a Panzer IV, crew hanging out of the hatches and asking directions to the nearest enemy tank. A vignette would be just the turret and the crew. Diorama would be some Marines and maybe a Humvee negotiating a house clearing in Baghdad, a vignette would be a section of wall, and a few marines doing an entry on a doorway. Vignettes can be as simple as maybe a soldier, leaning back on a chair at a table and as complicated as a bunch sitting around a fire. Dioramas can have just as much a wide margin of elements that make up the total.
There is a wide lattitude on folks interpretation of the two but suffice to say, one is the pie, the other is a slice of the pie.
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"