Thoughts are, it's best if nobody knows about your basement marijuana growing operation "gun vault". Yes, a vault is a nice security measure, but keeping things on the DL is the best first step.

In terms of expenses, your cheapest option would be to choose a room in the basement, and before putting on drywall, put up some security grade wire mesh. This will prevent anyone from just cutting through the drywall to get to your "guns".

You can also try for a hidden door. Although hidden doors are not usually built for preventing brute force takeover, they're cool, plus it keeps people from asking questions like "hey, whats behind that steel door?"

For good ventilation, as often required for "guns", you really want a high discharge of your exhaust such as via a chimney so any smell that comes from your "gun vault" doesn't cause the neighbors to complain. This typically involves a small utility set style fan with ducted discharge. Of course, with that much negative air, you'll need a dedicated makeup air line to prevent a draft and revealing your secrets.

Get as much natural light as possible to cut down on the power bill.

The key to all of this, of course, is a good floor plan from the start.