The 47-year-old is happy with how the Bianconeri squad is shaping up, but insists they must sign a top player if they want to become a threat in Europe
The Bianconeri are back in Uefa's elite club competition after winning the Serie A title - their first since 2006 discounting Calciopoli - in Antonio Conte's first year in charge.
Arsenal striker Robin van Persie and Napoli ace Edinson Cavani have both been linked with Juventus, and Vialli feels the Turin-based club must now focus on making a marquee signing.
"I like the squad that is being built, the club have already made good signings and if a top player arrives, I think they can be competitive, not only in Italy, but also in Europe," Vialli told Sky Sport Italia.
Juventus have been very busy in the summer transfer market, signing the likes of Lucio and Sebastian Giovinco as Conte looks to fine-tune his squad.