Folkloric vampires being mostly associated with Middle Europe, it's hard to imagine where or how Nelly Dean, a rural English servant in 1802, might have become familiar with the term. (There were fairies and such with vampire-like characteristics closer to home, but not under that name.).
But Emily Brontë, as an educated woman writing her novel in the 1840s, might conceivably be familiar with "The Vampyre" by John Polidori (published 1819), or perhaps, since she read German and French, and spent some time teaching at a school in Brussels, it's possible she could have been exposed to continental influences and stories—though I don't know specifically what any of those might have been.
The only thing that comes to mind, and it's a wild guess—the sort of guess one makes just to be saying something when one has no answers at hand, and only the vaguest ideas to draw on ... thus, just to be saying something I offer it for what little it is worth—is that it could maybe, possibly, perhaps have been the same book of German ghost stories read by Byron, the Shelleys, and Polidori during the year without a summer, which provided the inspiration for their famous (or infamous) contest, and eventually led to Polidori's short story, "The Vampyre", and Mary Shelley's novel, Frankenstein.
That's all I've got. If we are lucky, someone will come along with a better informed answer.