Schmidhofer had only one runner-up finish since joining the World Cup circuit in 2007, and even that super-G result was four years ago.
After crossing the finish line, the Austrian leaned back and screamed in joy when she saw she was 0.33 seconds faster than second-place Tina Weirather of Liechtenstein. Pre-race favorite Lara Gut of Switzerland, starting immediately before Schmidhofer, was third, 0.36 behind.
"It was surprising for me and I enjoyed the day," said Schmidhofer, who doubted she would win with Vonn and others yet to start. "I thought, 'Yes,' this could be a medal, but not a gold medal."
Like Vonn, Olympic super-G champion Anna Veith of Austria also failed to complete the undulating course.
Both Gut and Vonn prepared in less than ideal shape after crashing at the previous World Cup races in Cortina d'Ampezzo, Italy.
Gut, who won three straight super-G races to start the World Cup season, slumped in the finish area and shook her head after seeing she trailed then-leader Weirather's time.
"It was my first run after my crash, so I'm happy I didn't forget how to ski," said Gut, who now has five world championship medals but no titles.
Vonn plans to change equipment tactics for the combined event scheduled on Friday and her signature downhill event on Sunday.
"I am definitely going to have to duct tape my hand to my pole now," she said, adding that not to attach it was "too big of a risk."
After bad weather wiped out Monday's planned training session, all racers were getting a first good look at the course — and in a super-G race which rewards the ability to adapt at speed to an unfamiliar gate-setting.
Schmidhofer did just that and was the first to clock 100 kph (62 mph) at the fastest point of the course.
The result repeated a 2007 junior worlds race, when Schmidhofer edged Weirather for gold in super-G. In their careers, both have had season-ending knee surgeries after crashes at the classic Cortina course.
"We both had a tough way to go. It wasn't easy for us," said Weirather, who missed the past two Olympics because of injuries.
Weirather, also 27, won her first career championship medal at the same venue where Hanni Wenzel, her mother and an Alpine ski great, won slalom gold at the 1974 worlds.