The Spaniard was some-more than half a second forward of his nearest aspirant during a Circuit of a Americas.
Reigning universe champion Marc Marquez hold a early lead, while Honda twin Cal Crutchlow and Dani Pedrosa were also quickly in front.
But with 15 mins to go, Vinales pulled out a 2m05.462s, putting him 0.073s forward of Marquez – though he had some-more in a tank, posting an even quicker 2m04.923s to go 0.612s clear.
Marquez also softened late in a session, though usually by a tenth, withdrawal Vinales absolutely on top.
Ducati looked to be carrying a bad day during a bureau until a final 5 mins of a session, when Andrea Dovizioso softened from 12th to third, and Jorge Lorenzo from 16th to sixth.
Rookie Johann Zarco tender to finish fourth for Tech 3, followed by Aspar’s Alvaro Bautista, only inside a second down on a leader.
Pedrosa eventually staid in seventh, while Vinales’ team-mate Valentino Rossi had to settle for eighth.