In the light, many things become clear that in the shadows were obscured. The things that live in the shadows fear the light.
This is why vampires avoid the sun. By nature, they don’t actually burn up to ash and blow away. But they are convinced that is what will happen, and so they remain in the darkness out of fear. They remain convinced in part because every legend about them claims it is so. And who is willing to test what legends say?
This is the greatest achievement in vampire hunting, with burning success that has spread to every corner of the land.
Set the rules so that they dare not venture forth.
Seek them out where they shelter from the sun, and deal with them from a position of strength.
The real achievement is that the vampire hunters were able to unify the legends. This flies in the face of conventional wisdom. People tend to be fiercely loyal to their own traditions, and to resist changes as a chicken resists a cat. In the end, though, it was love that overcame their fears.
And now the vampires fear the sun.