The origins of the Assen dynasty are somewhat unclear, most likely were of Cuman origin, and most certainly not Vlachs. One thing is certain - they considered themselves rulers of Bulgaria and tried to prove a lineage to the ruling dunasties of the First Bulgarian Tzardom. There is strong evidence that Bulgaria maintained control of Vallachia at least until the end of Ivan Assen II's reign, and after that the region passed to the Golden Horde, but Bulgarian rulers maintained possession of cities north of the Danube right until the Ottoman conquest. Ivan Shishman, the last Bulgarian tzar, was able to defeat (and kill) Dan I and retake cities on the northern shore of the Danube even though Bulgaria was collapsing under Ottoman pressure. This shows that it was not until the 15th century that Vallachia and Moldova started to play any significant role in the region. I am not disputing the significance of Stefan the Great, but this is in the Pike and Musket scope and time frame, not Medieval Total Realism.