for instance, i would do a tier 7 delve , finish it . check on the great vault and it would show me the reward for a tier 6 and say…complete a tier 7 to increase this item level reward , but i just did that…
You have to do 2, then 4, then 8 delves at that level for it to increase.
Say you did a 5, and then a 7, you’d only get the ilvl reward for the 5, since that was your lowest completed of the 2. Do another 7 and see what happens.
I did a T8 bountiful with a key, finished with 6 lives, no deaths, and my vault preview says I only get Tier 1 rewards.
So 1 Tier 8 gets me tier 1 vault?
Are you saying i have to do 8 T8’s to get 1 T8 vault slot?
Didn’t work that way last week 
you need to do 2 of any tier for that slot to register in the first spot for the 2nd slot i believe its 4 and for the last slot i think its 8 not in game atm. for instance i only did 1 tier 8 last week so my first slot was tier 7 cause i had done 2 of those,
well now I keep getting DC’d and I have 2 of the flame caster guys in the starting room with me on a t8 waterworks
looks like I’m not finishing this one lol