Your input is appreciated for a debate. Would the 2 dons, Vito and Michael have been sucessful business leaders in a "straight" profession, and what would have helped them?

What an excellent question!
Yes, both would have been successful in "straight" businesses. In fact, both were: Vito was the largest olive-oil importer in America, and Michael was a successful investor.
The qualities that would have made them successful in "legitimate" business include: personal leadership dynamics; wise recruiting of subordinates; ability to plan ahead and think dispassionately about current (sometimes negative) circumstances. Above all: maintaining undeviating focus on their goals, which often entails ruthlessness--especially understanding the value of sheer, brutal power, and having no hesitation to use it. It's not an accident that the most successful leaders of big corporations--in Vito's time and in Michael's time--were ruthless with competitors. Perhaps they didn't have their rivals assassinated, but they used every tool, including semi-legal and illegal tools--to eliminate competition.
The models for the Mafia in America were the titans of the "robber-baron" era: Rockefeller, Carnegie, Morgan, Gould, Fiske, Harriman, Vanderbilt, etc. While 99% of Mafiosi were and are little better than common criminals, a few--Luciano, Costello, Gambino, Accardo, Lansky (although he wasn't a Mafioso)--would easily have been successful as "legitimate" businessmen.

Welcome to the site, and what a great question, as Turnbull rightly said!

Michael and Vito, if they didn't take up a life in the Mafia, would have made excellent business people. Vito had the smarts to plan things, while Michael had the smarts to make sure such plans were carried out. Using a non-legitimate example, take the Baptism scene: in the book, Vito planned much of it, with Michael fulfilling it after his father died. The same would apply to legit businesses too.

Mick