All members in your group must have the Stormhammer Quest in order to access the vault, if they don't have the quest they won't get the "update for the key" (Therefor you won't be able to access the vault).
You must have completed and turned in the collection "The Hammer of Below"; then speak with Commander Olaf Caltorsis Irontoe in the Butcherblock Mountains to start "The Stormhammer" signature quest.
You must be level 35 to receive this quest. *Note: You can receive it pre-35, but you can't enter Haunted Vaults 'til 35
Find the entrance to the lower levels of Stormguard Hall, aka the Haunted Vaults, at ( 196, 17, -94 ) Copy/waypoint 196, 17, -94. Do the Burning the Black Sun Grizzly Rug quest to uncover the vault. If you have done this quest before, you will have to do it again or you can pan your camera down and click the hatch underneath the rug.
Enter Haunted Vaults. Don't kill anything if you can avoid it, there is an 90 minute lockout timer if you do.
Click the red chest in the room on the 3rd door on the left hand side ( 179, 18, -60 ) Copy/waypoint 179, 18, -60.
Kill the 3 skeletons in the room on the last door on the right hand side ( 197, 17, -95 ) Copy/waypoint 197, 17, -95. Hail the ghosts spawning when they are dead. You will receive 3 sub-quests:
After completing the 3 subquests, return to the Haunted Vaults and kill the 3 ghosts in the throne room to spawn the quest NPCs to turn in the subquests. Each will give you part of a key, which opens the chest in the vault ( 179, 18, -60 ) Copy/waypoint 179, 18, -60.
Open the red chest in the vault to get the marching orders.
Enter Kaladim and proceed to ( -37, 25, -99 ) Copy/waypoint -36.5, 24.5, -98.5 where Xicotl (35^^^ or 36^^^) will spawn. Kill Xicotl. Warning: Xicotl has been reported to occasionally spawn as a level 75^^^!
If you do happen to kill the ghosts before checking the chest they do have a long respawn time of 8-10mins. You should wait for this since there is a lockout on this instance of 90 minutes.