How does it go from vacca in Latin, and vaca in Spanish, to mucca in Italian? I'm familiar with an "m" or "b" sound becoming a "v" sound in Gaelic (specifically mh, and bh having a "v" sound in the Scottish variant), but I have never seen the opposite in effect until now. How udderly fascinating.
The two words are unrelated, as far as I can tell. "Mucca" apparently comes from Swiss-German, according to the Treccani dictionary, which traces it back to imported Swiss cows: http://www.treccani.it/vocabolario/mucca/ "Vacca" exists in Italian, but is not as commonly used now because it has picked up some negative connotations: http://www.wordreference.com/iten/vacca
It's vacca in Italian. I studied Italian for 7 years in a top NYC private school, then took Italian literature, language and culture class paid for by the Italian government for non-Italians who study Italian. I've only heard Sicilians say "mucca." Duolingo does this a lot with languages.