To me Rio was the best defender since Maldini and Nesta in the early 00's. As others mentioned Nesta after 06/07 was struggling with injuries and at the time IMO Rio was the best in the world. Since then only Godin stands out, while Rio is comfortably better than Bonucci, Barzagli, Ramos, T.Silva etc.
As for comparing Rio and Vidic it's a hard one, considering Vidic fit the Premiership like a glove. Physical, gets stuck in and great in the air. Rio however was the organizer and main man at the back for me, which I always rate a bit more.