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I am a vegetable that is disliked by vampires and used to frighten them away
Garlic.
Garlic is indeed effective in repelling vampires. In particular, it is the chemical allicin in garlic that provides the effective chemical and odorous properties. As most people have noticed, garlic has a very strong smell, which in and of itself is not deadly or dangerous, but it does serve as a warning sign to vampires.