What seems to be the trouble? Where is the "dolly" you guys keep talking about? I see a pallet jack capable of lifting about 5500 pounds. I could put that in by myself, and I'm only 5'7" 180 pounds.
First you need to wrap the RSC with several moving pads, or blankets will do and tape them in place with packing tape. Next, position it in front of the step. Then tip it to its side through the door. Push the rest of the way into the room, stand it back up and use an actual dolly from this point. If you feel better about it, put a couple of 2x4 on the step to assist in protecting the side of the RSC that will be resting temp. on it. Or you could use a "Big Red" appliance dolly with straps, or use an electric stair climber........I do this for a living, and with things much more fragile than your vault. I drive a long haul truck, and specialize in electronics with an occasional household move. I move 3,000 pound mainframe computers into office buildings.....I lease to Mayflower/United van line.