The Vampire Bat is a Mob exclusive to the Hamlet DLC. Vampire Bats are essentially the Hamlet version of Hounds: They will attack the player periodically and will come in waves where their numbers will increase the more days the player has lived. They have 130 health and can be kited in the same way as Hounds. When killed, they have a chance to drop Monster Meat, a Pig Skin? and a Batilisk Wing?.
Apart from their periodic attacks, they can also be found sleeping in their den, a Cave Cleft. The Vampire Bat will seemingly stay asleep until something violent wakes it up, as a player's attack or an earthquake.
The player can tell when an attack is about to happen when swooping shadows of bats overhead appear, very similar to the Buzzards in Reign of Giants. They usually happen every 5 days, but it's mostly randomized. During the Aporkalypse, Vampire Bats spawn daily.
If the player enters a building when a Vampire Bat attack is about to happen, the Vampire Bats won't spawn until the player exits the room.
If the player waits in the building long enough the attack will be dismissed completely
An easy way to deal with Vampire Bats in the early days is to simply lead them to Royal Guards. However, caution must be taken to ensure that the Royal Guards don't burn down the environment, as they wield Torch at Dusk and Night.
As a bonus, the player can steal from nearby farms while the Royal Guards are busy fending off the Vampire Bats.
Take caution in Humid Season, as the fog greatly reduces the player's visibility, making kiting hard.