OR letting your personal feelings for a step son get in the way of cold hearted judgment. Tom was trained as a lawyer, and not as a crime boss. Sonny was the heir apparent. Even Michael was skeptical about a non-Sicilian consigliere way before he got into the family business. He even mentioned it to Kay during the wedding scene.
It basically was one of the few bad moves made by Vito. This and sending Luca Brasi on that spy mission is what almost got him killed. Clearly, by 1945, Vito was slipping. And his rivals, lurking like hyenas and sensing weakness, made their move.
