If you have 2 slots on heroic but only to fouth boss. And if your 3rd slot is a Gallywix kill in normal. All the slots could have loot pool from Gally and previous bosses i mean the 2 heroic and the normal one slot? or Not?.
Or the 2 heroic slots will have loot pool from up to 4 th boss only and the normal one from Gally and the previous bosses?
Bosses only count on the difficulty you’ve done them, and any kill counts not just ones that week.
So in this case if you’re 4/8 heroic and have normal gally, you will have 2x 4/8 heroic slots and one 8/8 normal slot.
If I understand your question correctly, once you kill a boss on a particular difficulty all loot for that boss and all previous bosses is unlocked for that difficulty.
If you’re 8/8 Normal and 4/8 Heroic your Normal vault slots will have all loot including Gally but your Heroic slots will only include loot from the first four bosses.
If you killed first 4 heroic bosses and only normal Gallywix (no other normal boss), then you would only have 2 vault slots, which are based on the heroic boss kill loot. You need 6 boss kills for the 3rd vault slot.
Or if you meant you killed 4 heroic bosses plus enough normal bosses to open the 3rd vault slot, then first 2 slots would be heroic boss based loot, and last slot would be based on whatever normal bosses you killed.