Let's say a creature (like a devil) is normally immune to fire damage. The unfortunate devil is within 30ft of a Grave Cleric and their Sorcerer friend.
The Grave Cleric uses their Path to the grave feature:
As an action, you choose one creature you can see within 30ft of you, cursing it until the end of your next turn. The next time you or an ally of yours hits the cursed creature with an attack, the creature has vulnerability to all of that attack's damage, and then the curse ends.
Is the devil still immune? Or are they instead vulnerable to the damage?
To put it another way, does the Path to the Grave replace the immunity with a vulnerability? Or does it add a vulnerability on top of the immunity?
Note: The linked question is asking "what happens if a creature somehow has both immunity and vulnerability" (presupposing that they can get into that situation in the first place). This question isn't making that presupposition, in fact it's explicitly asking if that presupposition can even happen.