SUNLIGHT IN BAROVIA

By the will of the Dark Powers, the sunlight is eternally blocked by clouds and mists in Barovia. Even in the middle of the day, the sun never fully shines. 
For the purposes of visibility, the sunlight is bright light, but it is not bright enough to affect a vampires' vulnerability to sun. 
However, it is true that Strahd and most of his foul creatures tend to venture out at night, and they are vulnerable to magically-created sunlight.