Based on this : http://avengersabout.tumblr.com/post/154645954795/missisjoker-avengersabout-that-moment-when

Basically, it can be either original!Graves turned into vampire by Grindelwald and Newt helping him cope with the new condition (and studying him at the same time because he is now a fantastic beast)

or a total AU where Newt hears rumors about a vampire in New York and tries to investigate, which backfires spectacularly when the said vampire becomes very, very interested in him.