I have never vacuum sealed a gun for long term storage, (ammo and other perishables though without issue). But, you have to figure that rust needs oxygen to form, and your gun being in an essentially oxygen free environment, it should work fine.

On a side note, I too keep a SS handgun stashed in the bathroom. If you think about it, its one of the most vulnerable places you can commonly be in your home if someone busts in uninvited.

Larry