When writing a RWBY vampire infection fanfic, start with a strong hook. Maybe open with a scene where a character wakes up in a strange place, already infected and not knowing how it happened. Then, research vampire lore. You can incorporate elements like a vampire's weakness to sunlight or silver into the RWBY universe in an interesting way. For instance, in Remnant, maybe vampires are weak against Dust of a certain type. Finally, have a clear plot structure. Decide whether the fanfic will be about finding a cure, or about the characters learning to live with the infection, and build the story around that goal.