Vonn, the 41-year-old who broke her leg at the Milano Cortina Olympics last month, had led the discipline from the start of the season with two wins and five podiums from five races.
Pirovano won by the slimmest of margins in Val di Fassa on Friday for a career-first World Cup win, and the 28-year-old repeated the feat a day later after crossing the line 0.01 faster than Austria's Cornelia Huetter.
Switzerland's Corinne Suter was third, 0.05 slower than the Italian.
Pirovano now has 436 points with Germany's Emma Aicher on 408 and one downhill to come at the World Cup finals in Norway on March 21. Aicher was 12th on Saturday.
"It's even more incredible than yesterday. I can't take it in. It's all surreal," said Pirovano, whose 0.01 advantage translated into about 28cm on the piste.
"The margin of one hundredth, twice in a row. I think it's payback for all those times the hundredths went against me."
Vonn's pre-injury points tally of 400 remained within the sights of Germany's Kira Weidle-Winkelmann (351), Huetter (344) and world and Olympic champion Breezy Johnson of the United States (333), who was fourth on Saturday.
"Winning the title was my goal... and I came painfully close to achieving it," Vonn commented in a lengthy Instagram post before the race.
"I clawed my way back to number one in the world after being retired for six years with a partial knee replacement and that alone was an incredible achievement I won't ever forget.
"Even though in a few days no-one will remember that I almost won the season title, I will remember...I just wish I had a chance to fight until the end to try and get it."
American Mikaela Shiffrin, the Olympic slalom champion and most successful World Cup skier of all time, who does not race downhill, retained the overall World Cup lead with 1,133 points to all-rounder Aicher's 1,016.
The women continue with a super-G on Sunday.
