Ok, to become a vampyou have to be bitten by one & then you're a "fledgling" until you take a human life, then you're a full vampire.
Erica was bitten by Jesse and then she killed Rory. BUT a few minutes earlier Rory showed Ethan & Ben his "cool hickey" he got from a girl at the party who was obviously a vampire. So he was ALREADY a fledgling not a human when Erica bit him! She couldn't "take his life" because he was already dead.So she shouldn't be a vampire from that. At least according to the logic of the storyline.