Vampire Mask

The normal route is Goodie Bags, which any enemy can drop while the Halloween season is on, so it comes down to farming a spawn and opening bags rather than hunting one specific monster. The shortcut is the vampirism secret seed, where the outfit is placed into ordinary chests during world generation. If you only want the costume and do not care about the world it lives in, that seed hands it over in the first few chests you open.

The Mask, Shirt and Pants all sit in social slots and change nothing about how long you survive. The shirt carries a cape drawn from the same back and front accessory layers the game uses for other capes, so it hangs behind the character properly instead of being painted onto the torso. On the older console and handheld builds that cape could double as a second back accessory, which the current versions do not allow.

The slicked hair, the high collar and the cape point at Bela Lugosi's Dracula specifically rather than at vampires in general. The equipped sprites were redrawn after release, so what you get now is a cleaner take on the original costume.