Born in Brisbane, Vermeulen began competitive riding in the Australian Superbike Championship in 1999, finishing eighth overall with a best result of fourth. Mentor Barry Sheene arranged opportunities for him in British Supersport and Superstock competition, which led to a World Supersport opportunity with Castrol Honda. After a modest 2001 season, he linked with Dutch team owner Gerrit Ten Kate in 2002, claiming his first poles and podiums and finishing seventh in the championship.
In 2003 Vermeulen became World Supersport Champion with the Ten Kate Honda team, winning four races and becoming the youngest ever champion of the series.
Ten Kate moved Vermeulen into the Superbike World Championship for 2004 aboard a Honda Fireblade, where he won four races and finished fourth overall — the only non-Ducati in the top eight. In 2005, expanded competition from Yamaha and Suzuki filled the grid, but Vermeulen secured his first pole position at Assen and took additional victories, ultimately finishing as championship runner-up after the cancellation of the Imola second race due to rain removed the remaining points margin he needed.
Vermeulen joined the Rizla Suzuki MotoGP team for 2006 alongside fellow young talent John Hopkins, having declined a Honda offer restricted to Superbikes. He scored his first MotoGP pole position in Turkey in wet conditions and delivered a strong wet-weather performance at Phillip Island, his home round, charging to second place after the field switched to wet tyres.
The 2007 season marked the high point of his MotoGP career. On 20 May 2007 at the Bugatti Circuit in Le Mans, Vermeulen rode from twelfth on the grid through a wet race to take his maiden MotoGP victory at the French Grand Prix. He followed that with third place at the British Grand Prix — again from twelfth on the grid in the wet — and pole position in wet conditions at Assen. In dry conditions at the United States Grand Prix he qualified third and finished second behind Casey Stoner.
The 2008 season was significantly more difficult. Suzuki struggled competitively and Vermeulen could not add a race win, although he secured back-to-back podiums at the German and United States Grands Prix. He finished the championship eighth overall. In 2009 his single top-five finish underscored the team's lack of pace relative to frontrunners, and Suzuki did not renew his contract for 2010, with Alvaro Bautista taking his seat.
Vermeulen returned to the Superbike World Championship in 2010 with the Kawasaki team, but a knee injury sustained in a crash at the season-opening Phillip Island round forced him to miss races. He reinjured the knee in a first-lap incident at Brno and sat out the remainder of the season on medical advice.
After retiring from racing, Vermeulen became an analyst and commentator for MotoGP on Fox Sports Australia. He married English model and photographer Toni Pinion in 2011 and the couple have two daughters. His racing numbers throughout his career always incorporated the digit seven, a tribute to his early mentor Barry Sheene.
Vermeulen's career encompassed three distinct world-championship environments — Supersport, Superbike, and MotoGP — in which he reached the very top level of each. His reputation as a wet-weather specialist, earned across multiple championships and continents, was confirmed by his 2007 French Grand Prix victory in conditions that exposed the limitations of many of his rivals.