Vermeulen left all in his wake in race one but was made to work harder the second time out, the Winston Ten Kate Honda rider winning an epic battle with the Yamaha Motor France rider Noriyuki Haga.
Vermeulen needed to win to stay in championship contention with the fellow Australian Troy Corser, who settled for points in both races on the Alstare Corona Suzuki.
After making an early break in race one, Vermeulen kept the chasing pack at a safe distance and was able to take it easy in the closing stages.
It was a lot less comfortable for the Ducati Xerox rider Toseland, who had to fight off the attentions of Haga before securing second. Haga reversed the grid position over the championship leader Corser as the Australian looked content to protect his series lead with fourth.
Andrew Pitt made it three Australians in the top five on the Motor Italia Yamaha and Corser's Japanese team-mate, Yukio Kagayama, rounded out the top six.
The Foggy Petronas team finished in formation with Garry McCoy in 13th ahead of the fellow Australian Steve Martin in 14th.
Chris Walker did not finish the race on the PSG-1 Kawasaki.
Toseland was dropped off the start of race two but fought back as Haga led from Pitt and Corser.
Toseland quickly moved into second and set off in pursuit of Haga as Vermeulen made equally impressive progress.
The lead then changed hands before a mistake by Toseland allowed both Haga and Vermeulen to get clear.
Haga looked to have made a decisive move with four laps remaining after setting the fastest lap of the race but Vermeulen then snatched the lead on the penultimate lap and held on by just a single bike length at the finish.
Corser eventually wore down Pitt and was able to take fourth at the finish behind Toseland.Reuse content