All I could say to this is lol. Orthodox Christianity is Orthodox Christianity, popular belief that Saint Elias is Thunder thrower is not basic concept of Serbian Orthodox Church, it is popular belief baised on some elemnts from ancient religion, nothing more.
Vidovdan is main point of Serbian Christian idenitiy. On Saint Vitus day (15th of June 1389) Orthodox Serbs fought Ottoman Muslim Turks on Kosovo Polje. Battle's outcom was inconclusive tactically, but strategically it was Turkish victory. Nevertheless Serbs continued to celebrate Battle of Kosovo as final affirmation of their Christian identity.
Those are superstition about mythological beings not old pagan practices. Karakondžula is Vlach's belief futher more not Slavic.
And exmaple of such sacrifice would be? Boiled wheat with sugar or honey? It is is used liturgically in all the Eastern Orthodox Churches as an offering of boiled wheat that is blessed liturgically in connection with the Memorial Services in Church for the benefit of one's departed, thereby offering unto God, as it were, a sacrifice of propitiation (atonement) for the dead person, and in honor of the Sovereign Lord over life and death. Serbs offer that for souls of their relatives who died in Christian faith. Slavas (literary celebrations) originated from times when Serbs lived in klans (arround adoption of Christianity), each klan got its Church, and all of them together celebrated patron saint of Church and klan, after migrations klans split but their keep Slava.