Hello there,
Last week I did a single Lv 7 Delve but the Great Vault tab would only indicate Current reward 584 (lv 1), complete a delve lv 2 or higher, etc…
And this Wednesday after reset that was what I indeed got.
Now I just completed my very first lv 8 delve, and I get a similar message on my pannel : reward 584 - (lv1), do a delve lv 2 or higher, etc…
Great Vault at 616 for delves requires you run Tier 8 or above, and to unlock all three Delve Slots you would need to complete 8 x Tier 8 Delve during the week up to reset
The Delve reward in the Vault is in the same category as the Open World Content. So if you only do one Delve, the lowest of the two will be taken; prob some World Boss killed leading to the minimal reward.
Do at least 2 Delves on level 8 to get one option of 616 gear in the Open World category.