Flag of Vicina, Dobrudja, around 1350, according to the story of a Spanish Franciscan friar: 

I left Constantinople and entered the Mare Mayor (Black Sea), proceeding along the coast of the left hand to a great city called Vecina (Vicina¹). Here nine rivers unite and fall into the Mare Mayor². These nine rivers make a great commotion before this city of Vecina, which is the capital of the kingdom³. It has a white flag with four red squares.
¹ - In National Geographic Vecina is confounded with Vidin, although the latter is far away from the Black Sea, and has nothing to do with the Danube Delta.² - The Danube Delta.³ - Possible refference to principality of Dobrotici of Dobrudja.
self made after National Geographic, October 1917 issue.
Alex:D