make a bedrock cube. have a command block designed to teleport a single player, the owner, connected to a button. Does this not work?
just realized when the last post was, sorry.
This is what I've been working on today (see post here: http://www.minecraft...0#entry22409195). I can't see a problem with it yet (with regards to security). Only problem is that it requires an op to build it and set up the command blocks.
I couldn't be bothered to read all 98 pages but my idea is if they get it wrong they are forced into a minecart and then half slabs are placed on top so they can't escape by clicking the cart, it move to fast (if done in a circle) to break the slabs and if youlog out and in again you are still in the mincart.
I know this is not the exact goal, but still, i'll post it:
With command blocks, make a security system with /clear and Gamemode 2 swapping, with a warning before the entrance, after that, there is a cube of bedrock with a code lock, when entered correctly, it teleports you inside another cube of bedrock with the vault inside.