Just did a t8 delve. Says vault slot opened. Look at it, says tier 1, and to complete a 2 to raise the item level
EDIT: so you need 2 of something to open the vault. first was a world boss, second was the 8. opens it a t1. did another 8, its an 8 now. 2nd is a t1 still or something. either way, meh