There is a great deal of vampire lore, Dracula, and other more recent authors, but IMHO, I think a lot of it is shaped by people's interpretations, and if an author, or person, doesn't like one particular aspect of the vampire lore, then they change it to something more inventive or something more to their liking.
I realize that some things, for example, the cross being a deterrent to vampires is something that is used in DS, but on the other hand, do we really know for certain that Barnabas, or vampires in general must "sleep" in a coffin during the day?
[spoiler]When Barnabas and Vicki were in that car accident just after Vicki's return from 1795, Dr. Lang wants to keep Barnabas in the hospital for observation. Julia tries to convince him to allow her to take Barnabas home, but he refuses. She then insists that all windows, and doors be completely covered so that no amount of sunlight can enter the room. Lang agrees.[/spoiler]
I may not have gotten all the details correct, but the gist of it is, that Barnabas was able to remain out of his coffin as long as no daylight entered the room he was occupying.
Chelsea Quinn Yarbro's Saint Germain series does this, despensing with the traditional fear of the cross, but keeping the idea that vampires are creatures bound to their native soil and so long as they rest on a bed lined with the soil from the land of their birth or line their shoes with it they maintain all their strength and powers even in daylight. So yes, sleeping in coffins is not compulsory.
DS however takes the view that as dawn approaches they must return to their coffin to sleep and that they become kind of dead and vulnerable at this time, hence the need for a Willie or a Julia. If this wasn't so, anyone coming to harm Barn at any time other than dusk would get a nasty shock when they open the casket. However, I think it is more simply a need to be cut off from any source of sunlight, hence Julia's insistance that Dr. Lang keep the curtains closed in Barn's room.