The warlock's Gift of the Ever-Living Ones (XGtE p. 57) eldritch invocation is described as follows:
Whenever you regain hit points while your familiar is within 100 feet of you, treat any dice rolled to determine the hit points you regain as having rolled their maximum value for you.
The description of the Vampiric Touch (PHB p. 285) spell states:
On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt.
How do these two interact? Do I roll for damage, and then gain 9 hit points?