I think Vlachs formed there, between Thessaly and Timok, at most between Thessaly and mountaineous Banat/Apuseni, and then at some point expanded elsewhere. they are most recent into northern Transylvania, Bucovina, Moldova and Dobrudja, where Romanian-speaking populations are just Romanianized Slavs and whatever other local populations they found.
also the initial Romance populations of the Balkans must have been quite mixed in origin and quite non-Italic, basically local Balkan and Oriental linguistically Romanized veterans with local women, so the Romance language isn't local per se to the Balkans, however it's interesting how it formed, survived, thrived etc, though speakers should understand they don't come from Rome, genetically :) I don't know elsewhere, but in Romania common opinion is they are Romans, with some Dacian in it, while in practice I suppose they are totally a different mix of origins (just a mix of Neolithic leftovers with a mix of later invasions leftovers, like Scythians, Celts, Germanics, over which other invasion leftovers came in each epoch, from Slavs after 500 CE to Steppe Turkics to Roma in the Middle Ages, to Greeks after 1700s to Jews after 1800s). I find this more real approach more fascinating to think where my genes came from than fantasize about Rome/Italic peninsula. just an opinion, don't shoot xD