Benjamin of Tudela, a Jewish traveller from Spain, visited Greece around 1170, when the Jews of Greece were all Romaniotes (Greek-speaking). Benjamin’s fellow Sephardic Jews only moved to Greece when they were expelled from Spain, three hundred years later. So whatever was going on, it was not because of any linguistic kinship between the Vlachs’ Aromanian language and any Greek Jews’ Ladino language.
Might it have been an appeal to Benjamin’s Ladino? No; language does not come up at all. The sum total of what Benjamin writes about them is:
From there it is a day’s journey to Sinon Potamo, where there are about fifty Jews, at their head being R. Solomon and R. Jacob. The city is situated at the foot of the hills of Wallachia. The nation called Wallachians live in those mountains. They are as swift as hinds, and they sweep down from the mountains to despoil and ravage the land of Greece. No man can go up and do battle against them, and no king can rule over them. They do not hold fast to the faith of the Nazarenes, but give themselves Jewish names. p.18Some people say that they are Jews, and, in fact, they call the Jews their brethren, and when they meet with them, though they rob them, they refrain from killing them as they kill the Greeks. They are altogether lawless. The Project Gutenberg eBook of The Itinerary Of Benjamin Of Tudela
Wherever Sinon Potamo is, it is two days walk from Gardiki, Trikala; so Benjamin was in Thessaly, where there is a substantial Vlach population.
As you can well imagine, historians have found this intriguing. That doesn’t mean it’s true; Benjamin of Tudela also claimed a Jewish Kingdom in Ethiopia, which recent scholarship is sceptical about (Desperately seeking the Jewish Kingdom of Ethiopia: Benjamin of Tudela and the Horn of Africa (twelfth century)).
The Vlachs in the area openly rebelled against Byzantium two decades later, and may not have been willing to accept Byzantine religious suzerainty, so Greek priests may have been in short supply in Vlach Thessaly. While most Greeks don’t use Old Testament given names, Cypriots do, Bulgarians did, and maybe Vlachs did too; a credulous Benjamin could well have run with that as evidence of something.
I think what’s likeliest is, the Thessaly Vlachs welcomed Benjamin as a non-Greek, were intrigued by his background, and told him some tall tales to impress him.