What is the kind of light vampires hate?

Light from the Sun; though it is often supposed by writers that the active frequencies (those that cause harm to vampiric creatures) are those in the ultraviolet range, providing convenient plot devices involving UV lamps.

Notably, a vampire's weakness under sunlight did not always result in explosive/combustive or rapid decomposition of their flesh; this is relatively recent evolution of the legend. Count Dracula, for instance, was perfectly capable of walking around in daylight, but was unable to access some of his higher powers, such as shape-shifting, flying, or transforming into a mobile mist.