This achievement can be found in level 1 "Trial By Blood".
At the start of the level turn left into the men's changing room then turn right and leave into the main room. From the exit of the changing room turn left again and into the room with the large statue. On the right side on a small table is a key card for the women's changing room.
Turn around and leave the room with the large statue and walk straight passing the main double glass doors on your right and into the woman's changing room. In the main locker section turn to the left to find the locker. Open this to find a key card for the men's changing room. Leave out the side door and walk straight into the men's changing room.
Directly in front of you will be the locker required. Inside you will find the VIP gold card. With this in hand go out the exit to the main room (same one you exited before going into the room with the statue). Instead interact with the keypad on the colourful wall directly in front of the exit. Once inside look on the left wall to use the VIP keypad.
Video Guide, starts at - 1:23