From my understanding Damage , Armor and Speed does not stack and goes by the highest multipler.

So in your above given example.

Bone Devil vanity set (300% "stacking damage")
Daily Blessing 5x

The damage output you should be receiving is the Bone Devil (this stacks damage which adds) to your highest damage multiple which is 5x only.

So you should have 5 + 3 = 8x damage.

All the other damage elixirs you do use do not come into effect , it goes by the highest multipler.

Only 3 sets on game where the damage does stack (add to your highest damage multipler which the maximum is 5x only - through 5th day Blessing combination elixir or 5x damage elixir you use - platinum version or elixir flask)

Bone Devil - 3x stacking damage
Exterminator/Chem Corps - 2x stacking damage.