I remember that the book is similar to "A Discovery of Witches" by Deborah Harkness, as I thought that was the one I was looking for until I started reading it.
I honestly cannot remember the specifics but I know that it does include a vampire and witch forbidden love.
I think the female lead is a powerful witch but isn't aware of the the fact. She might be a scholar or historian, something similar I think. She recently came into possession of a magical book as it just turns up in her house/apartment. It constantly does this appearing and disappearing act.
The vampire male lead is after the book or senses the power of it and is curious. I know he is constantly appearing at her home at night, most of the time she is sleeping I think. But I remember that she can sense him. Not sure if it was an ability of hers or if it was specific to him.
I remember the book being slow paced as it laid out the history and background in detail.