The My Big Fat Greek Wedding star co-wrote the script for the film with its star Tom Hanks and admits she and many others behind the scenes on the set were fascinated by Roberts' beauty and her famous grin.
Vardalos wanted to make sure her smile dazzled just like the Oscar winner's when she hit the film's red carpet premiere, so she signed up for GUM's 28-day PerioBalance challenge - and she was so impressed with the results, she's now a spokeswoman for the brand.
Vardalos says, "When she (Roberts) smiles, her head opens like a Pez (U.S. candy) dispenser. It's just the happiest smile. The DP (director of photography) is like, 'Dim the lights, my God!'
"I thought, 'I gotta do something, we're heading for the red carpet.' So I found out GUM brand. It's the PerioBalance challenge - 28 days... I love it so much. It's a probiotic... It's a lozenge that you put right beside your toothbrush, so you brush, you floss, you pop the lozenge into your mouth. By the time you make lunch for your kids, it's dissolved.
"It cleans 42 per cent of the plaque and it tastes like mint and it freshens your breath.
"I loved it because I'm busy; I don't have time to add another step to my life... But one step to cleaning my teeth is great."