Toad Brother, Turnbull and others, all very good points. I know that we are talking the movie version here. However I would like to add to this by bringing up the point that in the novel there is a part which indicates that Vito realizes that;

1) Sonny is too much of a hot head to run the family business.
2)Fredo, well you know, is not 'smaaaht' enough to do so.
3)Tom being a lawyer and a non Sicilian could not be given that role.

Which he realizes leaves only Michael to be the most likely and qualified heir to the throne. In realizing this Vito probably did not want Michael to lead the illegitimate life that Vito was basically forced to choose, so he began to plan to have Michael go to law school and peruse a career in politics so that, as you correctly said Turnbull, he could legitimize the Corleone empire.

But it was Sonny's small outburst that caused everything to change for Vito and his plan for Michael to take the legitimate path on behalf of the Corleone family.