There is no way to know that for certain, because Vito never had to experience the betrayal of a close relative. Like a jealous older brother, for instance. A betrayal that would have put not only his own life in danger but also the existence of the entire empire.

We tend to give Vito much credit as the 'kindly old man' and in GFII, dashing young Don. He really did not go through the type of transformation that Michael eventually did. He was a 'control freak' as well. This is so evident in how he chose to handle the killing of Fanucci; right from the dinner at which he tells Clemenza & Tessio that he'll 'take care of everything'...to the shadowing on the rooftops to the shooting (and raiding of the wallet) itself.

How ruthless was Vito? Pretty ruthless.