Well, it is really up to you.
Each year we try to come up with some sort of guideline (I use the term loosly) to help people focus, but it is not a written in stone thing. If you want to run with what the guidelines are, have it run something like this:
1st, answer what is left of Ft. Knox and why. Most cities and military installations were nuked...if Knox is left in any condition, explain why.
2nd, running with the above, you could explain which regular unit is left, what their mission was, and how they are accomplishing it right now. Even give an idea of how undermanned or pointless the mission is if you feel so motivated.
3rd, Work the vehicle in. However you want. Here is my example:
Ft. Knox got nuked early in the war when the opponents were still trying to use only small nukes. The museum actually survived intact, including the maintenance buildings, whereas the rest of the base was a nuclear wasteland.
The 200th MP Company is all that is left to defend the base's resources from local warlords. The civilian govt broke with the military and are traitors, so local govt officials are not to be trusted. There may also still be gold left on the base sight somewhere, and it is the duty of the unit to defend it to the last soldier. (also, if found, it would pay to get the unit...or select parts of it, taken to much more...comfortable, non-glow-in-the-dark, surroundings...)
Since the MP's need to patrol some areas that are very "HOT" they have been pressing into service any vehicle they can procure. There best "runner" so far is a old German WWII Hetzer, lined with lead (what could be scrounged) and given a very jury-rigged overpressure system. It is also easy to clean in the DeCon pit.
It has had it's main gun removed and replaced with a surplus Vulcan 30mm chain-gun...very effective crowd suppression. It has a special overpressure vent shaft to help keep the crew from being overwhelmed when firing the beast. Also, a remote control NVST AAMG has been added, for added protection that does not require leaving the sealed vehicle.
Hope that helps you out.