Well, unfortunately, Roth is not a novel based character, so we only have the films to go on. However, I just don't believe that Roth would have had any exposure to Vito's family. We see him about to work in Vito's crime family garage, but that's a far cry from his being exposed at all to Vito's kids. If Roth is a moniker for Lansky, then we can presume that it didn't take him long to step out on his own away from physical exposure to the Corleone kids. Also, given Michael's youth during the period in question, his being away at college later, and then his tour or duty, I don't see him knowing Roth either (until later) and surely not knowing if Fredo knew Roth. I just don't see it.