Bulgaria was a self-governing vassal since 1878. I dount the Ottomans would be willing to go to war to re-vassalize Bulgaria.Well, you did say that Bulgaria declared their independence. But I think with any outside help it would be impossible for them to keep it. Did they side with the quadruple alliance during the Great War ?
I revised a few things and added some details. (thanks to the person who made the base map) Here's the map: