Blood Promise by Richelle Mead

Blood Promise is the fourth book in the Vampire Academy series by Richelle Mead. Previously in the series, the main character, Rose, and her true love Dimitri made a pact that if either of them were turned into strigoi, the other would kill them rather than let them roam as an evil vampire. Blood Promise follows Rose's emotional struggle as she travels to Russia to hunt down Dimitri and fulfill this promise.
Blood Promise's opening week sales put the Vampire Academy series on the New York Times list for children's series, making its best rank to date at #2, just behind the dominant Twilight series.
In Blood Promise, Rose leaves St. Vladimir's Academy to go after Dimitri, who has become Strigoi. The only clue she has though is that he is somewhere in Siberia. After a meeting with an alchemist named Sydney, they travel to Siberia, where she eventually finds Dimitri’s family in the small town of Baia. While in Baia, she meets another shadow-kissed bonded pair, Oksana and Mark, and a mysterious Moroi man named Abe, who tries to force her to go back to St. Vladimir's. He eventually coerces her into leaving, and Rose agrees after falling out with Dimitri's sister Viktoria. She then travels to Novosibirsk with other unpromised guardians to stake out Strigoi in the hopes of finding Dimitri. When she does meet him, she is too stunned by his Strigoi appearance to attempt to kill him, and ends up being held hostage by him. He refuses to kill her... (Summary by Wikipedia)