Homebrew
Echolocation: The Vampire Bat can't use it's blindsight while deafened. Keen Hearing: The Vampire Bat has advantage on Wisdom (Perception) checks that rely on hearing.
Bite: Melee Weapon Attack: +3 to hit, reach 5 ft., one creature. Hit: 1 Piercing Damage. Special Note: If target is unaware of the Vampire Bat, it must make a DC 15 Perception Check in order to even notice the Vampire Bat feeding on it or being injured. This is due to the Vampire Bat's Neurotoxin on it's saliva that numbs the area it is feeding from. The Vampire Bat recovers 1 Hit Point for every point of damage dealt in this manner.