The folktales that vampires arose from were based off of human afflictions where as Dryads are spirits from a religion. While the lore may speak of physical creatures, it is not literal (much like the Christian Bible is not seen as literal, physical stories by many Christians); meaning that Dryads were never portrayed as literally physical beings while vampires were.
Spirits are not physical and would be subtle manifestations in a plane where there is less restriction on time, space, and all of reality than our own. We cannot isolate subtle energy because we cannot measure it; however, this is something we can do with physical energy, and thus, we have science. Science is able to let us know that vampires do not physically exist, though science cannot say spirits do or do not exist.