Well working in the grocery store wasn't really a start, it was more of a way Vito was able to live until Fanucci made Abbondando an offer he couldn't refuse. Vito's real "start" came when Clemenza got him to hide some weapons in his apartment and returned the favor by getting the rug from Vito from his "friend's" house. Vito then made fast work of Fanucci and founded Genco as the front for his truly lucrative operations. I am sure the family narrative that Mary always heard was, "Your grandfather came here as a boy, with nothing, started this olive oil company and got rich." That is how she came to say Genco is where Vito got his start. I do have a question though. Was the building she pointed out the same place where Vito gave that landlord all those problems in II? Ir looked like a different location, in which case the line should have been "That's where our grandfather expanded his business."