Here is one explanation of a Diorama as used for the IPMS Nationals.
BASES/DIORAMAS. Bases will be allowed in all categories and will not be considered in the judging except in the Diorama classes. A base may simulate the natural surface on which the prototype would be found; however, nothing other than that surface may be used. Aircraft and vehicles may rest on simulated ground or paving, and aircraft which need beaching gear or dollies may be so equipped. Ships may be displayed in water (no drydocks). The base must not be the predominant feature of the entry and must be of a size proportionate to the model. The Contest Chairman and Judges reserve the right to exclude oversize bases. The model may include primary crew figures. The addition of any other figures or equipment outside, or not attached to, the model (e.g., support equipment, foliage, shell splashes, or buildings) will make the model a Diorama, which must then be entered in the proper Diorama category. Dioramas are judged not only on the technical merit of their construction but also on the strength of the theme or story they present, so that if two dioramas were technically equal the one having the stronger theme or story would win.
A Vignette is considered to have 5 figures or less with or without 1 vehicle and no structures.
As described here, if you depict the crew on the vehicle, it can still be entered as the model and the figures, base are not judged.
AMPS has this rule: For Category X, a diorama is a scenic display of figures or figures and vehicles. Where a model with figures in NOT entered in this category, then only the vehicle shall be judged and the groundwork and any work on figures shall not be evaluated. Entries in Category X shall be evaluated on the 'story' their scenic entry tells. This will be accounted for as part of the 'Degree of Difficulty/Scope of Effort' criteria.
My opinion is that vehicles look better in their natural settings and a figure helps represent scale. Once you show 'action' or 'tell a story' you are in the diorama/vignette category.