Vos led off the final corner, but Bronzini was dragged into contention by her teammates Laura Trott and Peta Mullins, and launched herself to the line alongside the world number one.
Bronzini threw her arms in the air and Vos stretched out her hand in congratulations, but it was so close that at first the announcers weren’t sure who would get the verdict.
"That felt so good," said Bronzini, a former world champion who celebrated her 31st birthday last Sunday. "Any time you beat Marianne in a sprint you know you’re going to win."
Twelve months ago it was Trott who snatched victory for Wiggle Honda, but this time the 22-year-old Londoner played a supporting role, aiding Bronzini’s last-lap bid for the line, an effort the Italian was swift to acknowledge after the race.
"In the middle of the race I asked my teammates to make it hard and put in attacks to take the sting from Vos," said Bronzini. "What I did today was because of the support of my team.
"I think we made her tired by attacking as a team. That was our tactic today, and in the end it was a great sprint into a headwind.
Get The Leadout Newsletter
The latest race content, interviews, features, reviews and expert buying guides, direct to your inbox!
"She was ahead until 100 metres to go when I passed her, but she came back and I had to push hard in the final 50 and lunge for the line. The last time I was here for the Olympics it was not a good day, so this time I wanted to win badly. It is so amazing to win here in such a historic city and in front of these amazing buildings.
"I really like racing in the sun, and today was a beautiful day."
"It was so amazing to race here again, especially in such a great race," she added. "To be part of a criterium with all the world’s top riders was an amazing feeling."
Behind the two tearaway leaders, Lizzie Armitstead stole third place for Boels Dolmans from Eileen Roe of Starley Primal Pro Cycling as Trott took fifth ahead of her arch rival, Hannah Barnes of UnitedHealthcare Pro Cycling, last year’s runner-up.
Many had predicted Armitstead would be Vos’s closest rival, but the Briton admitted she was happy with third just a week after winning the Comonwealth Games road race gold in Glasgow.
"I had not really prepared for this because I’ve been concentrating on road races," said the 26-year-old from Yorkshire. "For me it was not about winning today but about having some fun."
