Di Vaio earns Bologna point at Napoli
The former Italy forward equalised in the 23rd minute, tapping in the rebound from a shot by Sergio Volpi, who appeared to handle after being gifted possession by a defensive blunder.
Christian Maggio had sidefooted Napoli ahead three minutes earlier after a German Denis cross from the right of the area.
Bologna's Davide Bombardini hit the bar with a powerful strike in a lively second half.
Napoli are joint seventh with Palermo ahead of the rest of the Serie A programme on Sunday but their hopes of being in next season's Champions League are fading as they now have picked up just two points in their last six games.
Lazio's ambitions of qualifying for Europe are waning as well after they were held to a 1-1 home draw home by third-bottom Torino earlier on Saturday.
Lazio defender Sebastiano Siviglia levelled with a header in the 75th minute after Stefano Mauri had attempted to score with an overhead kick to end a run of four consecutive league defeats for the Rome club, who remain 11th.
Midfielder Ignazio Abate had put Torino ahead in the 36th when he controlled a loose Ousmane Dabo clearance and found the net with a right-foot shot from the edge of the area.