Vonn, who had said her badly bruised right shin was bothering her after her victory in Wednesday's downhill, was last among the leaders to ski in the slalom run after being fastest in the morning downhill leg.
She was 0.07 seconds ahead of Reisch's pace at the first checkpoint but fell behind by 0.18 seconds at the second checkpoint, then missed a gate and lost her right ski before tumbling to the snow.
Riesch had a total time of 2 minutes, 09.14 seconds to beat Julia Mancuso of the United States by 0.94 seconds.
It was a second silver in as many days for Mancuso, who was runner-up to Vonn in the downhill race.
Vonn said the pain from her right shin "was killing me," after the twisting slalom run on an icy course pressed her ski boot against her bruises that have troubled her for two weeks.
"It hurts so bad," she said. "It's one thing to do the downhill but the super-combined is really tough on my shin. I tried as hard as I could."
Vonn struggled in slalom this season even before the shin injury affected her racing.
Anja Paerson of Sweden took the bronze, 1.05 behind Riesch. Paerson claimed her sixth Olympic medal just 24 hours after limping away from an ugly crash in the downhill medal race.
Top Canadian honours went to Shona Rubens of Canmore, Alta., who finished in 12th, while Emily Brydon of Fernie, B.C., ended up in 14th.
"It's really exciting," said Rubens. "It was a little tough in the downhill but I think I had a pretty solid slalom run. I'm happy with my day."
Not so Emily Brydon, who plans to retire at the end of the season and apologized for not putting on a better show for the Canadian fans.
"When I fail, it's not only I fail myself but I fail the people," she said.
Georgia Simmerling of West Vancouver, B.C., hurt her ankle in warmups and did not start the downhill.
Riesch, who trailed Vonn by 0.33 after the downhill leg of the super-combined event, punched the air and pressed her hands to her goggles in delight when she saw her time.
She came into the games as the biggest threat to Vonn's predicted domination of the women's Alpine events.
But the 25-year-old German was a disappointing eighth in Wednesday's downhill in her Olympic debut, and had an anxious moment in her downhill run Thursday.
She got too much air in a small jump before the halfway point and veered momentarily off course before correcting her race line.
-- The Associated Press