Talk to Kirac about the explosion in the Templar Laboratory
(2) Find Vault Key
Travel to The Fallen Courts
Find Vault Key from Dominus’ desk
Tip: Physically run around the borders of Oriath Square to find the (newly spawned and not showing on the area map, yet) entrance to The Fallen Courts.
(3) Unlock the Vault of Venarius
Travel to The Haunted Reliquary
Unlock the Vault of Venarius
Picking up the pieces locked door?
Search the Vault of Venarius Tips: Something similar was happening for me, but when I just immediately went to press the two key vaults one after the other(so it seems like it's a timed event) it opened for me and I was able to proceed. Maybe this will work for you.