The American clocked one minute and 22.82 seconds to snatch her 14th victory at her favourite skiing venue, her seventh in succession, but she confessed the effort had worn her out.
"I'm just shattered, I'm running on low battery," said Vonn, who collapsed at the feet of second-placed team-mate Julia Mancuso in the finish area.
With 56 victories, the four times World Cup champion is now the second most successful skier in history behind Annemarie Moser-Proell, whose record of 62 wins is in serious jeopardy.
But the downhill Olympic champion can already claim to have won more actual races than the legendary Austrian, whose record included seven combineds, which at the time were mere additions of a downhill and a slalom held separately.
Mancuso completed a clean sweep by the U.S. Ski team at the weekend by finishing second, 0.43 behind, emulating compatriot Stacey Cook, runner-up in both Friday's and Saturday's downhills behind Vonn.
Austria's Anna Fenninger took the last podium spot, 0.02 seconds behind Mancuso.
Tina Maze retained the overall World cup lead by finishing a solid fourth, 0.62 off Vonn's pace.
The Slovenian now leads the standings with 397 points, ahead of Germany's Maria Hoefl-Riesch on 319, while Vonn is back in third place with 310 points.
Maze saw her fourth place as a real satisfaction because, unlike Vonn, she does not like the Lake Louise course.
"The end section is so long that I kept singing to myself not to get too bored," she said.
Vonn did not have an ideal preparation for the weekend, suffering previously from a stomach bug which weakened her, and she said she would have to save energy this season to focus on her main goals.
"It's going to be a hard-fought season because Tina is really motivated and she can score points in every event," said the American, who said she considered skipping a number of slaloms this winter, especially the town races held in Munich, Zagreb and Moscow.
The World Cup circuit returns to Europe next weekend with a super-G, a giant slalom and a super-combined in St Moritz.
After her new show of strength in Canada, the question of whether Vonn should be allowed to race with the men remains.
Swiss coach Hans Flatscher jokingly said there was an excellent reason to let her compete in the men's ranks in the future.
"This way," he said, "there will be three available places on the podium in Lake Louise instead of two."
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- Julia Mancuso