For all the accolades Genco gets, there's little in the movie or book to indicate that he really was a true consigliere, i.e. counselor, to Vito. We never see Vito lean on Genco for advice and Vito doesn't seem to be the type to discuss strategy. I think Vito - like Michael - was really his own consigliere.

As Oli notes above, Vito's plans for the future make the selection of Tom understandable. First of all, his overly conciliatory nature would have been a nice counterweight to Sonny's temper. And his legal background and veneer of legitimacy (and fair skin/hair) would have helped the Family present a more "mainstream" face as they (and the Italian immingrant communit in general) assimilated to American culture.