Not very good analogies, my friend. Byzantine Sicily is separated from Magna Graecia by centuries.
As for the Goths and the Vandals, they were both supposedly East Germanic tribes, and Jordanes maintained that the Vandals spoke a Gothic language. Who knows how similar they were autosomally...probably pretty similar, but I like to wait for actual scientific data.
I'm not one of those people who are comfortable making authoritative pronouncements based on the fragments of writings we have from people whose knowledge of geography was rudimentary at best, and utterly confused at worst, and didn't even know that genetics existed.
I won't even get into detail about the peculiar credulity, from my perspective, with which some people approach the various origin myths that were created to increase the worth and dignity of some parvenu power. I mean, are we seriously going to believe Caesar's claim that his family descended from Venus? That's about as ridiculous, in my opinion, as the claim that some hut dwelling shepherds living above the Tiber were direct descendants of the lords of Troy.
Of course, I'm the tolerant sort...what people choose to believe is up to them...
And now, I think productive discussion on this particular aspect of the issue is about at an end.