I think you will find that bank vaults are dry enough, they are usually left open during the day time or at least opened often. I would be (and I am) concerned about home safes. Usually in the basement (damp) and usually not opened too often. When White Rose stores were in business, I got silica gel from them that had indicating crystals in them which were blue when the gel was dry then turned pink when you needed to bake it dry. I put the gel in an old plastic peanut butter jar which I had cut a large hole in the plastic lid, nearly the size of the lid, then cut a piece of non waxed non painted cardboard like cereal box, to cover the cut out and not let the gel spill out.
I don't know where to get that now but when I did get it from White Rose, it was in the craft section used to dry flowers. Maybe other craft shops would have it.
Tom