As we are discussing this exact issue for our upcoming show, this is an interesting topic.
The only thing that we have concluded so far is that it's a matter of size really.
Anyone remembers Marijn van Gils' "Godverdomme", the church ruin with a destroyed tank and................2 figures in the corner? It's not exactly a small diorama, but it would apply as a vignette if you only go by things like: max. 1 vehicle (a jeep or a massive modern Army Truck with trailer is both 1 vehicle, but considerably different in size) or max 2-3 figures.
If a show/competition decides for a need to diversify into dioramas and vignettes in the first place, keeping things simple would be a good point to start from.
You can define all kinds of things, up to the size of the represented elements other than the figures, but luckily our hobby is not rocket science. (Although some seem to think so
)
However, the message/story is another thing, I suppose. If you have no story to tell, why not stick to a single figure or stand alone vehicle in the first place? Saves you time and gives you the benefit of having time to finish more models.
My two cents.