The Vampire's Bite attack says:
Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount. – SRD 353
And a Necromancy Wizard's Inured to Death states:
Beginning at 10th level, you have resistance to necrotic
damage, and your hit point maximum can't be reduced.
– PHB 119
So what happens when a Vampire Bites a Necromancy Wizard? Does the Vampire regain Health because it dealt Necrotic Damage or does it not regain Health because it did not reduce the Wizard's Hit Point Maximum?