American Vu and Briton Hull share the lead on nine-under par, with Angel Yin and Kim Hyo-joo a shot behind, while Ewing is two shots adrift.
The 30-year-old Ewing, who was diagnosed with Type 1 diabetes shortly after gaining her LPGA Tour card in 2016 and carries a device with her on the course to monitor her blood sugar, had started the day at 10-under par, five shots ahead of the chasing pack following her six under-par 66 on Friday.
However, six bogeys in Ewing’s third round allowed her rivals back into contention for the fifth and final major of the women’s season.
“I don’t lose any confidence today other than I gritted my teeth when it wasn’t great, and I still gave myself a chance to win this golf tournament,” Ewing told the Women’s Open website.
“Certainly, would I like to go out free-spirited and shoot another 66? Yes. But the test was hard. And regardless of what the scores showed at the end of the day, I fought and gave myself a chance.”
Vu has had a rollercoaster 2023, but the 25-year-old was very much on a roll on Saturday as she constructed an impressive third-round.
In April, Vu won the Chevron Championship after a playoff win against Yin to seal the first major of her career, but then failed to make the cut in four LPGA events, including the Women’s PGA Championship and the U.S. Women’s Open.
The 25-year-old Vu hit five birdies in her second round of 68 at Walton Heath and she maintained that consistency on Saturday, as the American sunk seven more to take a share of the lead.
At the 16th hole, Vu came close to making an eagle before having to sette for her sixth birdie.
Hull’s round included six birdies, including three in a row at the 15th, 16th and 17th holes, before the Briton saved par on the 18th after finding the heather with her tee shot.
“Obviously it was so windy, but we dug deep,” Hull told the LPGA website. ““I struck the ball pretty well. I was pretty confident and felt good out there, and I’m pretty proud of myself.”
Yin is chasing a first major win after two runner-up finishes, including that agonizing playoff defeat to Vu at the Chevron Championship earlier this year.
A superb round of 67, ensured the 24-year-old Yin ended the day on eight-under par.