Lifedrinker and Warlock

the way i read it is that you dont have to expend a hit dice and regain hps ?

Once per turn when you hit a creature with your pact weapon, you can deal an extra 1d6 Necrotic, Psychic, or Radiant damage (your choice) to the creature, and you can expend one of your Hit Point Dice to roll it and regain a number of Hit Points equal to the roll plus your Constitution modifier (minimum of 1 Hit Point).

Yes, the additional damage and the healing are two distinct options. You can use one, both, or neither.