Vinales had been seventh and Rossi ninth at the previous three-day test in Malaysia.
"It was a difficult test, we struggled and we have to improve because we didn’t improve a lot compared to last year," the MotoGP.com website quoted Rossi as saying after the sessions at Thailand's new Buriram circuit.
"Especially with the electronics, so I think we have a lot of work to do."
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Vinales, who finished third overall last year, said the factory Yamaha team needed to improve in all areas with rivals Honda and Ducati setting the pace so far in 2018 after also battling for the title last year.
While Yamaha's main riders have found their bike lacking grip, the non-factory Tech3 team have looked much more on the pace, with Frenchman Johann Zarco second fastest at the Thai test.
Rossi is hoping for improvements at final testing starting on March 1 in Qatar, venue of the season's opening race on March 18.
"At the moment we’re struggling," he said. "But this is today. Maybe in Qatar it will be better."
(Reporting by Alan Baldwin; Editing by Kevin Liffey)
