Vampires are repelled, and injured by all things holy, and blessed. This is their plight as unholy demonic based entities of evil! Holy water is one example of such a religious artifact. That being water blessed by a Priest of any religion. The light of the sun is also Holy because it’s light is the one thing directly blessed by God himself. Naturally its holy rays are quite powerful, and cause the combustion of living dead vampire cells within seconds depending on the age of a vampire. The older the vampire the more resistant they are to sunlight. A First Generation Royal Vampire might be able to withstand direct sunlight for quite a few minutes. However the sinister steam rising from their skin would give them away to the general public.
In humans sunlight, or more precisely the Ultraviolet B light, results in the production of Vitamin D. However in vampires they react to the entire spectrum of the sunlight in a negative way. No specific component of the light causes them to be repelled by it. The supernatural scientific explanation is that a high enough concentration of holy photons hits the skin cells of these abominations of the dark shadows. Within these cells the vampire virus flourishes around, and intertwined with the once human DNA. When the photons come into contact with the demonic virus, and dark DNA it in essence cleanses it from the system. One would think this could be a cure for a return to human status. Unfortunately the key problem is the deep intertwining of the vexing virus with the DNA. The holy cleansing eradicates the virus, and takes all the DNA with it!
The entire cellular structure of the skin cells are destroyed exposing the next layer of their blasphemous body which in turn blasts into unholy fire. Like a domino effect it keeps taking out each successive layer the holy light comes into contact with all the way down to the skeleton. In the end nothing is left but unholy ash. The process can happen very quickly once the average vampire is exposed to sunlight. They have precious few seconds to find shelter as their skin burns, and unholy vapors rise from their body. Once they burst into flames the chain reaction is almost instant in the transformation to a pile of ash. Even heavy clothing isn’t enough to escape the field of photons.
Interestingly enough Moonlight is safe, and even soothing to vampires despite being a reflection of sunlight. The Moon has the unique property of neutralizing the holy light so it is harmless to virtually all demonic creatures of the night. Its most famous property is setting off the transformation of Werewolves which are another form of demonic based monster.