Vampire (Okay, are not real, but for the purposes of this thread) are magic. Magic deals with symbolism and mystic properties. The sun has a mystic property, as it is a symbol for good, or god, or whatever. Stars do not bear the same symbolism, and therefore are ineffective. Moon reflection is less symbolic of the Suns rays, therefore it usually doesn't effect vampires, though I do remember some places where it causes tanning. Depends on the mythos, but I think that a pretty good, broad explanation. In mythos where vampires are scientifically feasible creatures (Blade: Yes, I know that they are not scientifically feasible, but in THERE universe they are), then moonlight would have a slight effect, starlight even less, but it could very well be a matter of threshold.