The Monegasque will face Czech Jiri Lehecka, who rallied past Germany's Jan-Lennard Struff 3-6, 7-5, 6-4 to pull off his own surprise progress.
In Sunday's final, they play for only the third time with the spoils even as the 27-year-old Vacherot contests just the second ATP Tour-level final of his career after a maiden title at the Shanghai Masters 1000 in October 2025.
Vacherot was 4-1 down in the opening set tiebreak before taking six from the next seven points. He then broke the American at 4-3 in the second and closed out the contest.
"I guessed well," he said of Fritz's fearsome serve. "Even if you anticipate it you can hardly touch it."
Lehecka is three years younger but already has eight tour finals to his name. However, he has lost his last four championship matches, including two this season at the Miami Masters 1000 and the ATP 500 event in Tokyo.
Vacherot and Lehecka's most recent encounter came in the Tokyo semifinals earlier this month, when the Czech emerged victorious. Vacherot had won their previous meeting at the Paris Masters 1000 last year.