The Morlachs (= Black Vlachs) were a Latin-speaking population (no longer present) who spoke a Latin language related to Romanian, and which was quite different from the Dalmatian Latin used once by the inhabitants of Dalmatian coastal cities. This means that Morlachians were probably indigenous to the Balkans, as opposed to the Dalmatians of the cities, who were mainly of Italian descent (Roman settlers).
Around 1770, a Venetian intellectual, Alberto Fortis, visited the Dalmatian region and, among the other populations of the region, he also visited Morlachs and recorded some of their customs. Among other things, he also recorded the "aversion" of Morlachs in relation to veal consumption. The "disgrace" of this people from the consumption of beef is a "unique" custom that does not exist in any other, Balkan or European people. Apart from Morlachs, the closest geographic people with the same habit are the Sikhs of India.
When Alberto Fortis returned to Italy, he published his comments on the work of Viaggio in Dalmatia dell'Abate Alberto Fortis, which was published in Italy in 1774 and later translated into English under the title "Travels Into Dalmatia" and published in London in 1778. Let's look at what Alberto Fortis has written about this "unique" habit of Morlachs, the "abstinence" of eating beef.
The following text (translation) is from the work of Alberto Fortis "Travels Into Dalmatia", edition 1778, London, page 72:
“All sorts of domestic fowls ..... are heaped in prodigal quantities upon their tables; but VERY RARELY A MORLACCO EATS VEAL, and PERHAPS NEVER, unless he has been persuaded to do it out of his own country. This abhorrence to calves flesh is very ancient among the Morlacchi. St. Jerome, against Jovinian takes notice of it; and Tomas Marnavich, a Bosnian writer, who lived in the beginning of the last age, says, that the Dalmatians, uncorrupted by the vices of strangers, abstained from eating calves flesh, as an unclean food, even to his days”.
We can now take advantage of Fortis' remark in relation to the consumption of beef by Morlachs in order to possibly trace their origin, since their "disgust" of consumption of Beef is historically "unique" in Europe. It does not, therefore, meet any other people in Europe except Morlaku.
We will see this soon.
If you based this opening conclusion:
"The Morlachs (= Black Vlachs) were a Latin-speaking population (no longer present) who spoke a Latin language related to Romanian, and which was quite different from the Dalmatian Latin used once by the inhabitants of Dalmatian coastal cities."
upon Fortis' report, I must but ask - how?
Maybe you were thinking of report in Negri's Geography that was based upon interviewing several inhabitants of Morlacchia who, in the midst 16th century, were eager to travel from backward Venetian province (or from, so to say, wookoyebina) to more developed parts of Venetian Republic. These people told an Interviewer that they are of Roman origin hoping to make "an Italian connection". And they did not tell him that in the linger "quite different from the Dalmatian Latin" but in spoiled or pidgin Veneto, probably using their arms and legs to establish acquiaintance with an Italian gentleman. Their legends of their Roman ancestry are quite inconvincing, unplasuable, academicaly speaking - rubbish, because there are no other sources to confirm these claims.
Only Romans by origin western of Drina river are Dalmat(ian)s, Istro-romaniansi and predecessors of modern Italians.
Morlachs are not. Morlachs are the Seaside Vlach of South Slavic origin, or in South Slavic morski vlasi/Morovlasi, just as "Torlakians" (Torlak) are Torski Vlasi,
tor, standing for mountain path in South Slavic.
The Negri's observation was published 220 before Fortis:
Domenico Mario Negri: Dominici Marii Nigri Veneti Geographiae commentariorum libri 11, nunc primùm in lucem magno studio editi, ... , Heinrich Petri, 1557, pg. 103
"homines ... qui Latina (licet corrupte) inter Ioquendum non pauca proferant vocabula, seque Romanos fuisse, ibique prioribus temporibus in coloniam deducios pertinaciter asseverant"
"ljudi ... koji na latinskom (iako iskvarenom) narečju umeju da izgovore (proizvedu) nemali broj reči, koji sami bejahu Rimljani u pređašnjim vremenima dovedeni u koloniju, kako tvrdoglavo tvrde"
As opposed to Negri, Fortis tells quite a different story:
Albertro Fortis, 1774: – riječ Vlah nema nikakve sveze s latinskim,
– glavnina dačanskoga stanovništva, unatoč kolonijama što ih je ondje osnovao Trajan, kako svatko zna, bila od ljudi slavenskoga jezika, isto kao i narodi koji su tamo nagrnuli u kasnijim stoljećima;
– da slavenski osvajači, ako su imali dati ili ostaviti neko ime pobijeđenim narodima, ne bi tim narodima nikad bili dali ili ostavili ono ime koje znači plemenitost i moć, kako su nužno kanili, jer je to čista i prava slavenska riječ...
You say "When Alberto Fortis returned to Italy".(sic!) What 1770-ies Italy are you reffering to?