dnd 5e – What is the interaction between Gift of the Ever-living Ones and Dhampir Vampiric Bite ability

The healing isn’t rolled, so Gift of the Ever-living Ones has no effect.

Gift of the Ever-living Ones affects your healing rolls, not your damage rolls. In the case of the Dhampir trait, you roll damage, and then heal an amount equal to the amount rolled – the healing itself isn’t rolled, it’s simply set equal to the amount rolled by another dice roll. As a result, Gift of the Ever-living Ones has no effect, since the healing itself isn’t rolled for.

If, on the other hand, you had some sort of effect that heals you and then does damage based upon the amount rolled for the healing, then you would deal maximum damage with that effect, since the healing would be maximized as well. I don’t know if any such effects currently exist in the official content for the game, though.