I'm asking for proof that it was used as an "umbrella term for Serbs". 

Since beside Vlah, it says "Words[1] and phrases are pejorative if they imply contempt or disapproval. The adjective pejorative is synonymous with derogatory, derisive, dyslogistic, and contemptuous. When used as an adjective, pejorative has two meanings: (1) tending to make or become worse, and (2) tending to disparage or belittle.[2] When used as a noun, pejorative means "a belittling or disparaging word or expression".[3]"

THUS, I'm asking for you to prove your claims that it was an umbrella term.