Besides vreča, sometimes here they also say balh, and I can't seem to fathom it's etymology.

Couuld it be Celtic?
Old Irish and Gaelic bolg, Welsh bola 'bag, abdomen'; the Proto-Celtic, as well as the Gaulish form must look like *bolgo-s or something akin to that, the latter could be the source of Medieval Latin bulga 'leather bag' (the source of English bulge). Unlikely as it is, it's still possible.

On second thought, it should look like *blah if that was the case. Hmm…