****SPOILER****
search for the lightswitch in the darkness behind the door. Its around the middle left, 3cm in from just below the doorhandle, click around there you will find it eventually. Then you come to a dartboard, you have to throw the darts in the order of the special vault number in PS/SS (713), then you come to the safe and the number to get in is 302723, I don't know the significance of that yet though