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" (Therefore 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 ) /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.
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 ) /waypoint 179, 18, -60.
Open the red chest in the vault to get the marching orders.
Enter Kaladim and proceed to ( -37, 25, -99 ) /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^^^!