@Dema, we should take into account that there are two distinct Serb populations, autosomally.

One is Serbs from Serbia. These are the "real" Serbs as they origin from the population of medieval Serbia. During the Ottoman invasion some of them moved north into Syrmia and Banat. These Serbs hold significant paleobalkan admixture as they plot near Romanians and Albanians.

Second group is the Serb minority of Croatia, Bosnia and Herzegovina, popularly called "western Serbs". In the past they were called "Vlachs". However, autosomally they can’t be distinguished from their neighbors: Croats and Bosniaks.

So "western" Serbs probably can't be of "real" Vlach origin because they plot away from Romanians (Vlachs) and Albanians. However, it seem that they partly descend from Vlachs as they poses an elevated frequency of E1b haplogroup, comparing to Croats and Bosniaks. However, that "Vlach" signal was not sufficient to significantly change their autosomal profile.

@Bachus, your argument that - the “western Serbs” can’t be of Vlach origin because they are autosomally distant from Vlachs - is correct, but neither can they be of Serb origin, using the same criteria. ;)