As far as I know Vlachs are also known as Aromanians. From the very word, not Romanians. Meaning that they were some of the Balkan populations that did not get Romanized/Latinized like the Romanians. But they could have been of similar stock, not sure about that. Would be cool if Vlachs or Romanians in this forum would chime in.
Edit: Read some more on the net. Seems I have to read a lot more on the topic. Vlachs are truly fascinating. Fara Armãneascã ("Aromanian tribe"), interesting how many features Aromanian, Romanian and Albanian share. Fara in Albanian means seed. Meaning based on the nature of the world it has to date to the first farmers, and it being shared among various paleo Balkan people, has got to mean something about some proto common language/sprachbund from at least the neolithic/ farming related peoples.
Alas, ignore my initial post, seems that Aromanians were also romanized/latinized? at least from the wiki, so makes the whole nomenclature of a-romanian even more confusing.
All the Vlach groups are considered to share a common origin and to have split between 900 AD and 14th century. This includes Romanian, Aromanian, Istro-Romanian, Megleno-Romanian etc.