As for Dru, maybe the Master taught it to her? Or maybe when she and Spike were fighting sometime she became one of Dracula's chippies?
But why wouldn't the other vampires learn it? Darla never had that power, and she spent years with the Master and was his favorite.
I figure it's one of Dru's special powers, like her visions (though she had those even as a human) or her ability to see people's minds. The Master and Dracula are supposed to be special vampires, the Master is incredibly old, and Dracula has powers that nobody else has, like shapeshifting or creating castles (or illusions of them) out of nowhere.