For reference, from the Plane Shift: Ixalan PDF:
You can drain blood and life energy from a willing creature, or one that is grappled by you, incapacitated, or restrained. Make a melee attack against the target. If you hit, you deal 1 piercing damage and 1d6 necrotic damage. The target’s hit point maximum is reduced by an amount equal to the necrotic damage taken, and you regain hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
And from the basic rules:
Knocking a Creature Out
Sometimes an attacker wants to incapacitate a foe, rather than deal a killing blow. When an attacker reduces a creature to 0 hit points with a melee attack, the attacker can knock the creature out. The attacker can make this choice the instant the damage is dealt. The creature falls unconscious and is stable.
I'm a DM. I have a player who states that RAW, he can use this ability on knocked out (downed) enemy combatants at the end of combat for a 2d6 heal per combatant (and potentially 4d6 if they succeed their final death save on their turn). It's his intention that with party teamwork, a large majority (if not all) of the combatants will be knocked out, allowing for fairly substantial free post-combat healing.
While RAW this seems to be the case, to me it doesn't seem balanced. How does allowing this, or disallowing this, affect the balance of the Ixalan Vampire with respect to other races, and with respect to the challenges of a typical adventuring day?