in #dnd 5e a charmed creature by a vampire is a willing target to its bite attack.
However, doing anything harmful to that target allows it to repeat its saving throw.
Does the charmed creature also repeat its saving throw if bitten as a willing target? Yep! Damage from the bite is harmful, and can potentially snap the charmed victim back to their senses.

— Dan Dillon 👥 (@Dan_Dillon_1) March 11, 2020