When you start the Bonus Quest, Christopher will explain that he is a vampire. Cactus von Garlic enters the castle, intending to hunt Christopher. Christopher bites you, turning you into a vampire bat, and runs off. Katya instructs you to find a cure for yourself and Christopher.
Fly around the top of the room and you'll notice a hole in the wall. Enter it, and you'll be in the Attic, where you must dodge spiders to get the Garlic at the other side of the room. Leave the castle.
Cactus von Garlic will start shooting arrows at you. Enter the Second Mausoleum through the hole in the top and navigate the thorns to get the Wilted Wolfsbane. Go back to where you fought the wolves and enter the boarded-up area at the right side of the scene. In here, you can pick some Mandrake Root.
Fly back to the castle and enter the Laboratory. Cactus von Garlic follows you here, and intends to hunt you before you cure yourself with the ingredients you found. You tell him that he can help the vampires by curing them, and he declares himself to be "Cactus von Garlic, vampire savior" before running off. You comment, "what a weirdo."
Congratulations! You've completed the Vampire's Curse Island Bonus Quest!