I haven't read Bram Stoker's Dracula, but I have seen the movie. If the book follows the movie, then Dracula Untold should be very different. The movie adaptation of Stoker's Dracula was the story of Dracula long after he had turned into a vampire, and Dracula Untold seems to the story of how and why he turned, an origin story. I think it looks pretty good too, I'm looking forward to watching it.
I haven't seen the film, but read the book and yes, the book is all about after he is a vampire, and has lots of letters from a character called Lucy. It sounds like this one will be very different in that case.