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.
The story really begins when Genco dies. And with a few exceptopns the flashbacks in the novel and in GF2 are about the time before Vito becomes the big Don.
We can only assume that Genco was a great consigliere by the way he's referred to, like in Sonny's outburst: "Pop had Genco - look what I got." Just because we don't see strategic discussions between Genco and Vito doesn't mean they didn't happen.
We see Vito interacting with Tom even more often than with Michael. They discuss the Sollozzo business - before, during and after the wedding. And Vito seems to be really interested in Tom's opinion. And obviously Vito needs Tom to arrange the meeting of the crime families which must have been a very difficult thing to do.
