The world No.454, who secured his European Tour card via the Challenge Tour last season, fired eight birdies at Royal Golf Club on Thursday to finish a shot ahead of South Africa's Zander Lombard and Switzerland's Joel Girrbach.
The four-strong Australian challenge was led by Harrison Endycott, the PGA Tour Q-School winner who made up for a couple of early bogeys with five birdies in his last 11 holes to end up tied for 26th place on three under, five off the pace.
Pre-tournament favourite Rasmus Hojgaard eagled the ninth, his final hole, to card a five-under-par 67 matched by Scotland's Scott Jamieson and England's Marcus Armitage, who made a hole-in-one on the 12th.
"I just tried to hit as many greens as I could and just play solid the whole day and try to putt at a great speed and if I make some putts it's great," Vaillant, who has missed the cut in his first two starts this season, said.
"I'm fighting as hard as I can. I like it when the conditions are really tough. I just feel that my game is fit for that. I fight for every shot.
Hojgaard has finished 11th, 11th and second in his first three events in 2024, but revealed he was struggling with an illness during his 67.
"I'm very low on energy," he said. "I was trying to locate where the bathroom was on the course to make sure I was in reach.
"Sometimes, you just have to deal with that and I did that very well today."
A shot behind Endycott on two-under was his compatriot David Micheluzzi, while their fellow Aussies Jason Scrivener and Haydn Barron were both one-under.
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