Pass the sync-plate phase and enter the vault up until the first chest. stand in front of it, and turn to your left (so chest would be to your right.) then slowly walk backward until you are on the edge of the platform (chest is now in front of you and to your right slightly). drop down, and time your glide/jet/jump right, and you'll see a passage underneath the platform the chest is on. This leads you down to the templar, and the spot you come out on is pretty interesting. The area is beautiful, has a lot of verticality to it (me and my group went as low as we could, and there was a lot of ground to traverse down there.) No idea what lies down here, if this is a hidden chest area, an area bungie had planned to do something with and then scrapped, i dont know. point is, i found out it was there. and now you did too :]