Vacherot completed a fairytale run at the tournament on Sunday, rallying from a set down to defeat his cousin, Frenchman Arthur Rinderknech, in the final and secure his first ATP title.
"It's extraordinary," de Massy told Reuters.
"As a sports achievement, winning a Masters 1000 title, the first in singles for our federation. History has been made."
With the win, Vacherot rocketed from number 204 in the world to number 40.
"It's unreal what just happened," Vacherot said after his victory in the Chinese city.
"I have no idea what's happening right now, I'm not in a dream, it's just crazy. I'm just so happy with my performance these past two weeks."
Monaco had previously celebrated two Masters 1000 crowns won by Hugo Nys in doubles. De Massy said that Vacherot's run in Shanghai - which she flew out to witness from the semi-finals - capped a "tremendous year" for the tiny nation's tennis programme.
She credited fellow Monegasque players for setting the tone.
"Valentin keeps saying Hugo and Romain (Arneodo) inspired him. They opened the way, and now he's triumphing in singles."
'Remarkable young man'
The federation chief, who also presides over the Monte-Carlo Country Club where Vacherot trained as a junior, described the 26-year-old as "a remarkable young man" whose speech after the final reflected a tight-knit, family-style setup that blends the national team with up-and-coming juniors.
"We always try to include the kids in everything," she said, adding that junior players wear the same tracksuits as the Davis Cup team and often share courts and mealtimes with the seniors.
"Valentin definitely gave them a dream this weekend," she said.
De Massy said the Prince of Monaco, Albert II, followed the matches closely and congratulated Vacherot at a homecoming event in Monaco after the final.
She also placed the victory among Monaco's biggest recent sporting moments, citing Charles Leclerc's Formula 1 successes and past national achievements in sailing and bobsleigh.
"There aren't many times we can say we have an athlete winning major international events. We're extremely proud," she said.
Monaco, with a population of around 40,000 and an area of about two square kilometres, is "punching above its weight" in tennis, de Massy said, noting the men's team's current Davis Cup ranking of 44th among more than 160 nations.
"For a country this small, that's a remarkable achievement. I hope this is just the beginning."
