Interesting little excerpt from a piece in the Southeast European Times:

The origin of Vlachs, like that of the linguistically related Romanians, remains an unresolved puzzle. Both peoples are considered by some to represent descendants of Roman peoples in the Balkans, while others argue that they descended from Romanised colonists. Romanian culture was influenced by the Slavs, while Vlachs, originating south of the Danube, show Byzantine and Greek influences.

Historians call them Macedo-Romanians, and they themselves use the name Aromanians. The consensus among linguists is that Vlach and Romanian are variants of the same Latin-based language (another, Dalmatian, died out in 1898, while Istro-Romanian is spoken by a few thousand people in Croatia). Toponyms, such as Mount Durmitor in Montenegro, attest to the continued presence of Latin-speaking peoples in the Danube and mountain regions.

The name "Vlach" comes from Gothic, and originally meant "foreigner", and later "speaker of Latin idioms". German tribes used the name "Welsh" for the Roman population of what eventually became known as Wales, while the Romans of Southern Belgium were given the name Walloons. Hungarians to this day refer to Italy as "Olaszag," or "Land of Olachs" -- their version of the term. During the Byzantine Empire, several Vlach territories were recorded, but they seldom became powerful states. The Vlachs' greatest success occurred during the Assan dynasty (1185-1258), when they established the second Bulgarian Empire or the Bulgarian-Vlach state.