But in any case, the important part is how well the origin of the rules surrounding vampires fits vampires as literary constructs. That's the strength of the Molag Bal creation myth: a vampire is, pretty much by definition, a being that must attack and physically violate other intelligent beings for sustenance. Vampires make a lot of sense as rape metaphors, with blood-drinking as the stand-in for sex. Of course these nightmarish creatures who must ravish or corrupt the living to survive come from the King of Rape.
Why are vampires typically burned by the sun? Because vampires are cowardly creatures of darkness, who hide in the shadows to stalk their prey and cannot bear the light of truth. It's symbolic. The idea that vampires are vulnerable to sunlight is not arbitrary, but has to do the symbolism of the sun. Vampires are particularly vulnerable to fire as well. Why? Because fire is often associated with cleansing. It burns away all impurities. Thus particularly spiritually impure or corrupt creatures fear the flames. Similar elements apply to other traditional weaknesses of vampires. Running water? They can only cross still or stagnant water, because running water is clean. Why are vampires harmed by holy ground, or why do they recoil from holy symbols or texts? Same thing: their corrupt nature cannot bear truth or sacredness. Garlic? ...okay, I can't really think of a good reason for garlic. But whatever.