Has anyone considered the name Wallachs? It's a bit easier to pronounce than the Bl or Vl spellings of the demonym in English, and could easily arise if Wallachia is the dominant or unifying state, resulting from a possible confusion/actual etymological relation between the name of the country and the name of the people. In my opinion this is the most likely option, but it might not be if Transylvania remains separate for long enough that "Wallachia" is seen to solely comprise the borders of the OTL state.