If no one has answered you already, Vlach is a very complicated surname.
I am by no means an expert on this area, but have gained my knowledge on Vlachs through internet and encyclopedia searches. I encourage you to do the same.
The term Vlach is derived from a Germanic word meaning 'Romans' or 'Italians' and was used to describe ethnic groups. Other forms of this word are Wlach, Wallach and Olah.
Some would say that the Vlach people, based on their Latin derived languages, are the descendants of Roman colonists in Eastern Europe, the Balkans and Greece.
Today the northern Vlach people live in northeast Serbia, northwest Bulgaria, Romania, Moldova and a few in the Ukraine and Hungary. Southern Vlachs live in Pindus and Macedonia.
The origin of the Vlach people remains a mystery, and gives rise to disputes regarding the ownership of Transylvania.
Romanian national identity refers back to the Vlachs, and modern day Romanians use the Vlachs' cultural and linguistic ties to Rome as a way of differentiating themselves from the surrounding Magyar and Slavic ethnicities. Magyars (Hungarians) claim Vlachs are not the direct ancestors of Romanians and that they did not traditional live in Transylvania. All these opinions have more to do with current socio-political issues than any historical facts.
In terms of the origin of your specific family surname, this could be a variety of reasons. Since the word Vlach is used in most Slavic langauges to describe an Italian and in others to mean shepherd, your family may have been given this name for a variety of reasons. Only further research of your family's origins and occupations may reveal this.
Hope this helps.
Anna