So i recently discovered that one of my teenage sims, the child of a sim that I created and turned into a vampire and alan stanley, is a pyromaniac....she is also a vampire from birth but i thought it was strange that she has the pyro trait when no one in her family has that trait or is in the firefighter career.

My guess is, if you've installed a mod that makes the "hidden" traits available in Create-A-Sim (Never Makes a Mess, Can Salute, Burglar, Rocker, Pyromaniac, etc.), then sims created by the game also have the possibility of randomly rolling one of those traits in their personality. That might explain how your sim has that trait.