As many have said, it's hard to imagine that Vito would have allowed a brother's resentment to get to the point Fredo's did.

While he softened as he aged, it's not out of the realm of possibility that, while younger, had he truly believed that his brother was endangering the Family's/family's safety, he would have used whatever means necessary to stop the threat.

Two major differences between Vito and Michael:

1. While others disagree, I do not believe that Michael really believed Fredo was an ongoing threat. He killed Fredo out of vengeance and because, as he told Lamberto in GFIII, "he injured me." Vito never would have done this.

2. Had things gotten to that point, Vito would have believed it was his own failures as a Don and brother that led to the situation and would have redoubled his efforts to make sure his relationships with his wife and other family members were strong. Michael was incapable of such reflections, at least at that point in his life.