i always considered a diorama an 'action' scene where there are many figures doing different things on a bigger scale than i vignette
a vignette i considered 1-3 figures 'pausing' in time so to speak....thats just me....
diorama-bigger scale, action
vignette-smaller scale, moment froze in time
i'm sure there is a much better definition, but this is how i classify the 2