After finishing an 11 delve my vault shows that I have only completed a tier 1.
Did you also complete a world event like the theatre troupe or awakening the machine, etc? The first delve slot in vault requires 2 to complete, and it picks the quality from the lowest.
So if you did one T11 vault and a world event, the world event is what counts towards that slot. Do another T11 or T8 and you will get the max reward (616) for that slot.