The Frenchman penned a new two-year deal last month.
Sucu told La Gazzetta dello Sport: "I was very afraid of losing him. He promised to stay, and he kept it. He's the central figure in our work.
"He'll stay here for at least the next two years. We're working towards a team that's increasingly aligned with his style of football. Vieira builds ideas. It was a huge surprise for me to see how he spoke to the players and staff.
"He used exactly the same words as me when I spoke to my colleagues. It seemed impossible for someone who grew up three thousand kilometres away. And then he complements (sporting director Marco) Ottolini well."
Sucu also owns Rapid Bucharest and added: "I hope we can do better than last year. Rapid has already become an Eastern European club that many want to play for, thinking it's a quicker route to Serie A.
" And it's also interesting for our young players here, who instead of starting in Serie B, can immediately join a club with a winning mentality."