Tomassino's death was pure coincident. Remember, that when Calo brought Michael the news of his death, he asked Michael to command him. When Michael passed control of the Corleone family to Vincent I think it would be naive to believe that suddenly Michael was powerless and without influence.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."