Douglas Harper's Etymology Dictionary
"member of a Latin-speaking race of the Balkans, a Walachian or Rumanian," 1841, from Bulgarian vlakh or Serbian vlah, from Old Church Slavonic vlakhu, a Slavic adoptation of Germanic *walh (source of Old English wealh) "foreigner," especially applied to Celts and Latins (see Welsh).
Wikipedia
Vlach ( Serbo-Croatian: Vlah; plural: Vlasi) ( Ottoman Turkish: Eflak; plural: EflakÃĒn) was a social class within the Ottoman Millet system, composed largely of Christian nomadic and semi-nomadic pastoral populations of genuine Vlachs and Serbs.
Usage examples of "vlach".
The considerable Vlach or Ruman colony in the Danubian districts dates from the 18th century, when large numbers of Walachian peasants sought a refuge on Turkish soil from the tyranny of the boyars or nobles: the department of Vidin alone contains 36 Ruman villages with a population of 30,550.
Serbian and Bulgarian comitadjis, Greek andartes, Albanians and Vlachs, .