the vapor barrier will not be enough. The issue is not really the safe rusting. the issue is that the temperature changes will cause the safe to swing in and out of the dew point. That will cause condensation to form on metal objects inside the safe.
I would suggest multiple layers of wood. A pallet on a box for a start. Then be sure that what ever area you put it will have the least amount of temperature swing.
I as i said i recommend a pallet on a 4'x4' box. The inside of that box should be filled with rigid foam. Then you also need to have enough mobile desiccant.
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