IMHO, that's the point of a diorama: to tell a story. A vehicle on a base with some groundwork, perhaps a building, a figure or two standing around isn't a diorama — it's a vehicle model on a display base.

But if the figures are looking at a map and there's a fallen-down direction sign next to them, or if the figures are lounging about but a couple of enemies with guns ready are sneaking around a corner, or the vehicle is stuck and the figures are trying to dig it out, then it's suddenly a diorama. However (to tie this in with the topic  ) you would have to arrange them all so that this is obvious from the side you consider the primary side it will be viewed from.