Conversation so far:
What is the etymology of vlachs?
The word "Vlach" is etymologically derived from the ethnonym of a Celtic tribe, adopted into Proto-Germanic *Walhaz which meant "stranger".
Why were the considered strangers?
signify "stranger, foreigner" also in the Balkans, where it in its early form was used for Romance-speakers, but the term eventually took on the meaning of "shepherd, nomad".

Question: Was it considered derogative?
Self-contained rewrite: Was Vlach considered derogative?