In the original script, Jeremy kills Vicki and that is what original shot. Jeremy is forced to choose between saving the love of his life or his sister, and he chooses his sister. A few weeks later, Kayla Ewell was brought back from Los Angeles to Atlanta to re-shoot Vicki's death scene only Stefan kills Vicki. The makers chose to have Stefan kill Vicki because it makes him more of a hero since he saves Elena. There was also the question of would Jeremy know to actually stake a vampire in the heart.