That's an interesting question, now that I think about it. "Day" really is just a function of the Earth, and not of the Sun. So from that point of view, it makes perfect sense that vampires in space wouldn't need to sleep, as there is no such thing as "Day" or "Night" when you're away from a planet's surface.
It's not unreasonable to consider it "continual daytime" in space, though, in which case vampires are screwed. You could say that vampires can only operate in the shadows of natural bodies that are spherical due to their own gravity (ie. planets, moons, really big asteroids), which is a fairly general description of the "Day/Night" phenomenon, and even works for tidally locked bodies. Maybe there's something magical about natural spheres; circles are kind of the antithesis of the rectilinear cross-shape, and are well-known in magic.
This situation also renders unshadowed space very much like an ocean, in the traditional vampire sense.
You'd have to define some sort of threshold where sunlight becomes weak enough that it counts as general nighttime, otherwise you run into the starlight problem. I'd just apply the inverse-square law to the damage, and make the cut-off the point where sunlight does less than a maximum of 1/2 HP per melee (which would be a little ways past Saturn's orbits). This would lead to a really creepy situation of vampires lurking in the dark gulfs between stars, preying on unsuspecting outer system and interstellar travel.
That said, it must really suck to be a vampire on a planet in a binary star system.
"Then you can simply spead the ground dried corpse bits amongst the plants as needed." - Sir Ysbadden
"There weren't many nukes launched in the apocalypse, so the nuclear winter wasn't that bad." - Killer Cyborg