Wilting Juve beaten at home by Bologna
Seventh-placed Juve, enduring a second dreadful season in a row having initially battled back so well from a 2006 match-fixing demotion, also lost 2-0 at lowly Lecce last weekend which prompted a public scalding from president Andrea Agnelli.
Coach Luigi Del Neri was spared the blame for that defeat given it came after a win over champions Inter Milan, but he may now be getting twitchy in his first term in charge as Juve have gone through four managers since returning to Serie A in 2007.
Fit-again Vincenzo Iaquinta hit the post just before the break for the hosts but Di Vaio netted four minutes into the second half after the inconsistent Felipe Melo lost the ball.
The much-travelled 34-year-old striker then dribbled through a hapless defence to grab a second and power Bologna up to 11th despite the club having three points deducted earlier this term for payment problems and having gone through a turbulent sale.
Juve were without suspended goalkeeper Gianluigi Buffon after his red card at Lecce while central defender Giorgio Chiellini was again employed at left-back in a change which has puzzled fans of Italy's best supported and most successful club.
Leaders AC Milan host second-placed Napoli in an eagerly-anticipated clash on Monday with the rest of the league programme on Sunday.