Is there a compelling reason not to keep the instruments in their cases? If you aren’t a dealer, or a collector of valuable and important instruments, a vault seems overkill.
I wouldn't try to store violins in one.
I guess a gun safe is OK if you want to protect and hide the violin. However if you want to be able to see it there are other large vertical glass & wood cabinets or glass & metal enclosures that protect from the elements but can keep your instruments visible.
Seriously, I don't think the requirements for gun storage and violin storage are at all identical. Constant climate, especially constant humidity, is priority number one for musical instruments, safety against thieves for guns.
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