Well i think the reason why vampires in skyrim don't burst into flames since its a different vampire strain, well thats what I think, hey at least they don't sparkle in the sunlight.

Oh and in bram stoker dracula vampires don't burst into flames in the sunlight here is some info about the book.
It is also notable in the novel that
Dracula can walk about in the daylight, in bright sunshine, though apparently without the ability to use most of his powers, like turning into mist or a bat. He is still strong and fast enough to struggle with and escape from most of his male pursuers, in a scene in the book. Traditional vampire folklore does not usually hold that sunlight is fatal to vampires though they are nocturnal. It is only with the film Nosferatu that the daylight is first depicted as deadly to vampires.
So skyrim vampires are pretty close to folklore when it comes to the sun.