See my post here: cs.finescale.com/…/157403.aspx
As to the difference between vignette and diorama, that really only comes into play at a competition where you want to keep a level playing field. Generally speaking, however, a vignette is figure centered with several figures and usually does not have a vehicle though there should be some interplay among the figures. . In a diorama, there’s a more in depth story being told and may have several vehicles and extensive architectural elements. But, in the end, it is how the sponsoring group defines vignette and diorama that matters. Check with the rules and or the head judge for clarification.
A single figure or vehicle on groundwork is not typically considered either vignette or diorama.