> A vault doesn't have to be impregnable, only visibly difficult to steal from.
I'm pretty sure a vault in this situation has to be really impenetrable and definitely not just visibly difficult to steal from.
What you're proposing is the exact situation they had. The result didn't look good.
To obtain better security, the motivation of the participants needs to be analyzed. Perhaps this is a problem best solved through obscurity, with two competing security teams installing two totally separate alarms systems, each standing to lose if their alarm is defeated. Or perhaps skip the alarms, and rely on threats of deadly force.
But although any new insurer will properly demand the technological improvements illustrated here, it would be foolish to believe that technology alone will keep out a dedicated team of insiders with full access to the system. Real world security is not equivalent to public key encryption where security can be established by 'proof'.