I typed out the entire IPMS rule...then my computer locked up!!!
In a nutshell,
You can put the tank on a simple base that can simulate the natural ground (only) on which the tank is located and you can use all normal crew figures for that vehicle and the entry will be judged as a vehicle. Bases cannot be oversized and must be proportionate to the model. You can also use any non-terrain type bases that are simple and proportionate in size to the model. These can include, but are not limited to, mirrors, wood, or plastic.
The inclusion of any figures other than the normal crew for the primary vehicle, any foliage, or equipment not attached to the vehicle (support equipment, oil drums, etc) will make it a diorama.
As wbill76 said, non-IPMS contest rules will vary, so check with the Contest Director in advance to see what their rules are.
Hope this helps...