In particular I'm interested in building a walk in vault.  It could conceivably be inside an existing room (so a modular approach), or it could be a free standing structure as I have the space.  If the latter my preference would be  some type of reinforced concrete solution, however I see there are other 'hybrid' preformed slab solutions that claim to be 'as good as' actual concrete.  But I am pretty new to this all and just curious if anyone has any advice or knowledge on the issue.

I haven't built a vault, but I know where a few massive bank vault doors were stored a few years ago, after the demolition crew removed them from the banks.   You'd need a crane to lift them, easily 10,000lbs or more, just for the door and frame.

It would probably be easier to fabricate a frame and door from scratch and add a few high security padlocks.   Doesn't look as impressive, but will still slow down a pro burglary crew with cutting torches.  The amateurs with hand tools wouldn't have a chance.   The padlocks we used on nuclear weapons bunkers in the USAF would withstand cutting torches.   They were made of tungsten I believe.

I'm fixing to sell/scrap a bunch of structural steel soon.   Shoot me an IM if you're interested.

Another option is to find a large ex-govt safe, cut the door and frame off and use that.   You could lengthen it or just raise the frame high enough off the floor to provide clearance for your head.

I've played with plenty of safes, and the commercial ones are ridiculously easy to break through with $30 of chinese tools.

I'll be putting a few safes on the EE shortly.

I may be interested in that steel.  I'm just trying to figure out what is economical at this point, I need at least 100 sq. feet of storage, preferably 200.  With those kind of storage requirements I'd have to buy quite a few $4-5k safes, and at some point it makes sense to just build a bigger vault is my thinking.  But I could be wrong.