Aside from the well taken positions that Tom would act as a counterweight to Sonny, it is important to remember that the Corleones' main asset was political and judicial protection which allowed them to control labor unions and gambling. They
didn't have a lot to do with prostitution...that was Tattaglia's big deal, and Vito wanted to stay out of the drug rackets.

Tom, a lawyer, would be the obvious go-between with the politicians and judges.

Additionally his loyalty to Vito was unquestioned.

Finally, I think Vito really loved Tom as a true son, more so than Michael or Sonny did.