Tom had been consigliere for only 3 months. He took over when Genco died on the evening of Connie's wedding.
Solozzo probably would have moved against Vito after Vito said no, even if Genco had still been consigliere. Tom thought of himself as an unfit consigliere, and that Genco would have smelled a rat, after he learned that Sonny had been murdered. But, I'm thinking that Genco would have figured that Solozzo, as a Sicilian, would take Vito's "no" as a personal affront--depriving him and his people of a living--and would have predicted that Sol would move against him.
I have to agree with Dontomasso. Tom was a good consigliere, just not in wartime. My thought is that when Puzo said Hagen chided himself for being "an unfit consigliere," it had more to do with the guilt/grief from Sonny's death.
I put some of the blame at Vito's feet - he was slipping. He should have never had Fredo as the backup driver. More importantly, in the novel Vito asked Tom's advice about Sollozzo proposal before they met with Sol. Tom advised Vito to do it as in the movie, but most importantly realized that the "no" would have serious repurcussions.
Tom was stuck in a tough situation. He had to balance Vito's strengths against Sonny's many weaknesses. All without offending both men that Sonny was a terrible heir to run the family in the event that Vito was no longer in charge (volunatarily or not).
