The over-arching point that Raetin is trying to convey (and I think the OP is missing), is that you have no way to know whether the information you will be getting from your "vault" is going to be correct. Things just don't usually work like that, in dreams. You can get some insight, and you may pick up things that you weren't consciously aware of, but - at some point - you will get false information. So it's not a very accurate way of cramming for a test.