Metal plate backup solutions are great, but by storing them in a vault, you're basically saying it's a valuable item ^{[it shouldn't attract that kind of attention]}!
^{- I prefer to make multiple metal backups and hide them in different locations that they can easily blend in.}

Been there, done that and I've lost half of them

I'd also recommend periodically checking your backups ^{[create another one if you can't find one of them (this usually happens if you're really good at hiding them)]}.