The vault museum is located in the Hard Rock Store, which sits across the street from the actual Hard Rock Cafe. It can only be visited when accompanied by one of the guides, and there are tours twice an hour, every hour. But just ask and if you're lucky you might just get a private guided tour. As did I.
It's quite a small display and I wasn't down there for very long, but I figured that an old bank vault filled with a bunch of guitars once owned by famous rock-stars is definitely worth a nosy around. Especially if it's free.