OK...true Scott G makes the rules, but he does listen to us within reason.

I am one of the organizers behind our local show here, and in our rules (not affiliated with any organization like IPMS, AMPS, NNL, whatever).

Tank/vehicle is on a simple wooden plaque, it can be entered in it's native category.

If you lay down some sod/sand on said plaque, because that constitutes ground work, it is considered a diorama, whether or not it tells a story, and whether or not if it has a figure or not.

Armored vehicles with their crew, as long as the crew members are in/on the tank, it is still considered an Armored entry, but as soon as one of those figures touches the ground, or, you add say an infantry figure on the ground talking up to the commander, then it is considered a diorama. With that being said, if you came with a T34 with half an infantry squad hanging off the tank....I would consider that a diorama as well, since the tank doesn't require those infantry to operate, (maybe for their survival, but definitely not operation).

Looking up IPMS Canada National rules I did come across this in their rules however:

And while that is the Canadian IPMS designation, it pretty much mirrors the US IPMS designation for dioramas.