How does Vampiric Touch interact with Resistance/Vulnerability?

Question

As a follow on from Can a Spell like Vampiric Touch drain life from a creature in 0 hp?

If I use Vampiric Touch against a creature resistant or vulnerable to necrotic damage, what happens? How much do I heal?

Answer 1

Vampiric Touch says (PHB p. 285):

On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt.

And resistance and vulnerability is on p. 197

If a creature or an object has resistance to a damage type, damage of that type is halved against it. If a creature or an object has vulnerability to a damage type, damage of that type is doubled against it.

Given that D&D 5 says what it means and means what it says, in theory anyway; the point is that as far as possible you should take the words at face value. The "damage dealt" is the damage after any resistance or vulnerability effects.

For example, assume the attack hits and the 3d6 roll is a 10.

1. Against a normal creature it deals 10hp and heals the caster 5.
2. Against a resistant creature it deals 5hp and heals the caster 2.
3. Against a vulnerable creature it deals 20hp and heals the caster 10.

To my mind, this is in keeping with a spell that sucks the "life force" from another creature to bolster yours; if the "sucking" is more or less effective then so is the healing.

Damage dealt vs damage taken

First, I disagree that these are different concepts - they are the same thing from opposite points of view just as my left is your right when we stand face to face.

Notwithstanding, resistance or vulnerability affects "damage of that type". So it affects damage: "damage dealt" and "damage taken" are both damage so they are both affected if you insist on distinguishing them.

Answer 2

The healing effect of Vampiric Touch is actually independent of damage resistance or vulnerability. This is because:

On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt.

In D&D 5e, damage dealt and damage taken are 2 different things.

The damage dealt is the result of the calculation done by the creature who is causing the damage. The damage taken is the result of the calculation done by the creature that receives the damage. The use of these terms is consistent throughout the rules.

The example given in the Damage Resistance and Vulnerability section (page 197 of the PHB) demonstrates this particularly well:

For example, a creature has resistance to bludgeoning damage and is hit by an attack that deals 25 bludgeoning damage. The creature is also within a magical aura that reduces all damage by 5. The 25 damage is first reduced by 5 and then halved, so the creature takes 10 damage.

So, the attacker does their calculations, and arrives at "an attack that deals 25 bludgeoning damage". Then the creature being attacked does their own calculations, and "the creature takes 10 damage".

For another good example of this distinction, consider the Acid Absorption trait of the Clay Golem.

Acid Absorption. Whenever the golem is subjected to acid damage, it takes no damage and instead regains a number of hit points equal to the acid damage dealt.

The Clay Golem has immunity to acid damage listed in its regular immunities, so it doesn't need this ability to take no damage from acid-based attacks. Only the distinction between damage dealt and damage taken makes this ability have any effect. There are 3 other creatures with the same ability for different damage types, so this is unlikely to be an oversight.