If any person from the Dalmatian coast has read what you stated about Marco Polo’s origin, he is probably already dead of laughter.
In the local culture the word Vlach (or locally: "Vlay") means: a man from the hinterland, more correctly a mountain shepherd.
You see, wherever Marco Polo came from, that place must be near the sea. Korčula is the island of sailors, captains, fishermen and agriculturists. It is believed that Marco Polo family originated there before they moved to Venice. However, nobody can say that for sure.
"Morlacks" was the Venetian catch-all term for people who lived in the Dalmatian hinterland regardless of their ethnic origin and language. People who lived along the coast were called Boduls. These two groups never lived in "harmony".
Both Vlachs and Boduls were speaking Croatian since the medieval times. However, in some coastal enclaves Dalmatian Romance was preserved until 19
th century. Dalmatian Romance has nothing to do with Daco-Romanian. It is a separate branch. There is only one small bilingual enclave in Istria mountains (Ćići) who partly speak the Romance language that can be related to Vlachs.
Genetics studies on Croats don't show any significant genetic footprint of indigenous Vlachs in present day Croatian genetics. It is more likely that only some of Croatian “Vlachs” were of “real” Vlach origin. Others were just Slavs who accepted Vlach lifestyle and therefore started to carry Vlach label ever since. Almost no people today call themselves Vlachs nor Morlacks.
The "stećci" tombstones you mentioned have inscriptions (epitaphs). The language of the texts is Slavic. The people who were buried under the stones had Slavic names. It is just a speculation that these people were Vlachs.
Btw, do you know what Vlays use to say about maritime cuisine? “If fish was any good, wolf would be a diver”.
