Rio de Janeiro beat Bell Pottinger-advised Madrid bid in the final round. Pre-vote favourite Chicago, backed by Hill & Knowlton, and Tokyo, which used Weber Shandwick and Jon Tibbs, were eliminated earlier.
Vero had been working with Rio 2016 on their bid since June 2008. Vero initially worked on a bid by Doha before the city was eliminated at the first stage last year.
Agency founder Mike Lee was formerly director of communications and public affairs for London's 2012 Olympic bid.