Iceman999 : if you research the net you will find the original script for Godfather III by Puzo. They remain close, and Tom is with Michael till the end.

I am not sure Vito ever lost trust in Tom. He told Michael that Tom will understand their plan& why he is out for the time being. Not having Tom around was a luxury for Michael, that he could do because Vito was there to help him. But at any rate during the commission meeting Tom was the consigliere and he was the one who knew about the complete affairs of the family. It would take him years to teach all that to Michael(in the movie we skip this time).