So, let's imagine for a moment that two Fiend-patron Warlocks are fighting. One uses its Fiendish Resilience to get resistance to fire damage, but the other one casts the Hallow spell with the Energy Vulnerability effect to impose vulnerability to fire damage.
The description of the Fiendish Resilience feature:
Fiendish Resilience: Starting at 10th level, you can choose one damage type when you finish a short or long rest. You gain resistance to that damage type until you choose a different one with this feature. Damage from magical weapons or silver weapons ignores this resistance. (PHB 109)
And the extra effect of Hallow:
Energy Vulnerability: Affected creatures in the area have vulnerability to one damage type of your choice, except for bludgeoning, piercing, or slashing. (PHB 249)
Which trumps which in this scenario? Or do they cancel out and result in standard (1x) damage?
Also, bonus points:
- If imposed vulnerability/resistance trumps innate vulnerability/resistance, how does it interact with immunity?