Most companies don't, in fact, have vaults. Most consider protos to be trash and they're dealt with accordingly. Very few companies treat prototypes/builds/archival seriously.

Some developers have kept prototypes as keepsakes, or because they had some on their desk when they were packing up to move, etc.

It usually takes years (yes years) of patience to get a particular prototype from a non-collector.

Then there are collectors like me who have protos they themselves found or bought from other collectors.