In the world of folklore, vampires are indeed portrayed as creatures of the night, with a common belief that they retire to their coffins or some kind of protected lair during the day. Now, whether this is considered 'sleep' is subjective; it's depicted more like a state of rest or dormancy. This concept changes from one story to another.
In 'Dracula', they return to their coffins to rest, while in 'Twilight', they don't sleep at all. So, it really hinges on the author's imaginative twist!