Well, true. But surely it should affect them to some degree, no?
I guess it depends on whether it’s the heat or the actual gamma photons that they are weak too. Heat would make more sense in this case, because sunlight reflected from the moon is weak enough to not keep people warm on earth (also, in many versions, vampires are also highly susceptible to fire)
But we’ve also seen vampires be able to go outside during the day during cloud cover.