Vampires are a complicated creature to define. The contemporary kind draws heavily from sources that drew from Dracula and Dracula drew heavily from Carmilla. It is often said that Dracula is mostly based on myths of the strigoi from Romanian mythology. They dranks the blood of their victims and could turn into animals. They were not, however capable of turning others, they instead came from dead sinful people.
I based my vampires rather explicitly on Dracula. Olivia in particular shares many of his strengths and weaknesses, including him not being harmed by sunlight. My other vampires took on different traits of vampires throughout pop culture.
Olivia believes that different vampires receive different abilities due to the different species of vampires all inbreeding, as even when dealing with immortal monsters, she prefers thinking through science, despite this not really matching what she sees happen.
My personal explanation would be that a vampire loses what will hurt them the most.