Move to the panel on the left, this details what each vault contains.   Make a note of which vault contains the Sovereign's Sceptre, Orb and Imperial State Crown (the details are also in your PDA).

Move to vault 2 and enter the code you have worked out (reveal), this will open the door to reveal the Sovereign's Sceptre and Orb

Take the Sceptre and Orb and put the replicas in their place.

Repeat the procedure with the vault containing the Imperial State Crown (reveal code).

To leave the vault (the doors closed behind you) use the palm print reader at the centre of the doors.

Go back to the computer on the wall and login, go to the Log Section and DELETE the entry for your hand to remove any evidence of you being there.

Retrace your steps to the lift and go up one floor (i.e. press 1 on the lift control).   Turn left and walk to the end of the corridor.   Go through the door and turn right, through the door at the foot of the stairs.   Turn left and walk to the end, face right and click on a single book lying on it's side.  A secret door will open (the one with the rusty bolt)

Use your torch and go down the steps into the sewer.