I think that what nailed it for DeNiro is that he was essentially re-creating a character already played to perfection by by a different actor (Brando, who I thnk had ALSO studied method acting under Strasberg).

The fact that DeNiro had to play the younger Vito based upon Brando's older version and more than successfully pulled it off...made his performance all the more impressive.

If he had played the role without the still recent memory of Brando's portrayal, DeNiro may still have gotten a nomination but might not necessarily have won the Oscar.

As that 'Inside the Actor's Studio' fellow accurately put it...DeNiro 'completed a portrait'. And for that alone, he completely deserved the win.

Apple