Partially the vampire bat is responsible. It really does drink blood (mostly animals’, but still), and stories of the furry little monsters reached Europe by way of the narratives of Renaissance explorers. And then there is the medieval tendency to depict demons with bats’ wings, as opposed to those cute little cherubs, who sported eagles’ or doves’ wings. Probably this is the origin of the correlation between vampires and bats. And since bats only come out at night, it stands to reason, doesn’t it? Witches had their familiars, turning themselves into black cats and such, so why should a vampire not be able to transform into something conveniently winged, like a bat?