Hi Rookie!!  LOL....I suppose this one depends on what the judges want it to be!!   Ken Knows about that...the third Category......Dio-vignette!!   ;D

Typically, my take, Diorama tells a story and would most often have one or more figures.

Vignette is a simple version of this...with or without a figure in a much smaller setting.

This is where the line get blurred sometimes....the diorama was originally created with the meaning "small scale replica of a scene” the essence of this is actually portrayed in both.  Dioramas are to depict a scale version of either real or fictitious events, this would of course be with or without figures....now, more often than not, a figure of figures is required by most judging competitions.

A vignette is a snippet if you will of the diorama.  A very small, quick look at a scene.  It is a bit strange that this term is used as it is widely used in the literary community referring to a scene from a book, movie, play, etc....  With that though,, again, a vignette is an interpreted differently by the judging community.

Blah blah blah....Big scene....little scene, kind of sums it up!  Figure can be applied to either, just a Diorama would tell a little bit more of the story or scene, if you will, of your message you are trying to convey!!

Hope this helps a bit.....I am sure there will be a few more enlightening comments to either side of this.