Lost Girls (2009)

Stefan explains his and Damon's past with Katherine to Elena. In 1864, they both loved a woman named Katherine Pierce, who then turned both of them into vampires. Meanwhile, Damon spends the day trapped inside the Salvatore Boarding House with Vicki. They feed off of each other and he changes her into a vampire. Stefan discovers this and attempts to persuade her to die instead of completing the transformation into a vampire by feeding on human blood. Logan uses the pocket watch that tracks down vampires and finds Stefan and Vicki in the woods. Logan shoots Stefan with a wooden bullet, thinking Stefan was attacking Vicki. Damon saves Stefan by attacking Logan. Vicki is overcome by the hunger and feeds on Logan, killing him and completing her transformation. Stefan tells Elena that Vicki has become a vampire, and Elena decides to keep this a secret, but not without ending their relationship.