Ah, the great dio-vignette debate.
Our club worked hard at trying to figure this out. A vignette typically does not have vehicles, may or may not depict action and usually has 5 of fewer figures. These may be mounted or on foot.
A diorama tells a more compex story and typically has at least one vehicle. The amount of action depends on what the story is.
A figure in or next to a vehicle does not automaticaly constitute a diorama though a sing;le figure and a vehicle might. I would not clasify a single, buttoned up vehicle charging across the desert a diorama. More than one would, however, make a dio.
One figure by a lamppost would not be a vignette. A lady of the evening being approached by a pilot would be a vignette. Add a building in hte background with windows and action in the rooms, and you have a diorama, even though there are no vehicles.
Consider: Andrea figures all come with some sort of related base. None of these are vignetes out of the box. Verlinden has been releasing a number of sets, such as "Glory of Rome" and "Viking Raid" Out of box, these would be vignettes. Andrea has a kit, I bellieve called "Apache" in which an army supply wagon is being chased by an Indian on horseback. Out of box, this is a diorama.
Confused? You won't be after the next episode of Soap. #:-)