It is undeniable that Bulgarians, Vlachs and Cumans were in extremely close relations during that period. As for Vlachs, I belive they were not that different from the Bulgarians (after all, Vallachia was part of Bulgaria for centuries before the Byzantine conquest, and the population there spoke the same language as in Moesia), and the main reason why so many Greek chroniclers refer to the armies of Kaloyan consisting of Vlachs might be as a synonim to Bulgarians with a slightly negative connotation - Greek chroniclers tend to describe their enemies as Barbarians, and often use use any word that may have offensive meaning to their foes. The Cumans are a little more complicated, and there have been speculations (or we should say a hypothesis) that ethnically, they (The Western Cumans at least) were somewhat similar to the Bulgarians, before the latter mixed with the Slavs. Whatever the truth, the quick inclusion of the Cumans into the Bulgarian society and politics suggests some ties. Anyway, this is really diverging from the topic and might cause a riot among our Romanian friends, which is really not my intention.
It will be interesting to see how the mod handles faction emergencies and the establishment of Crusader states, such as the Latin Empire. I understand this would be tough to do, but I trust in the creativity and ingenuity of the modders.