Vettel, who will attempt to capture his eighth pole of the season later in qualifying, set a fastest time of 1:36.733 on the option tires. Webber was 0.203 seconds off the pace in second with defending race winner Lewis Hamilton down in third.
Nico Hulkenberg enjoyed a brief stint at the top of the charts before being bumped down into fourth. Romain Grosjean was fifth fastest followed by Jenson Button, Nico Rosberg, Sergio Perez, Valtteri Bottas and Adrian Sutil.
Both Ferrari drivers struggled in the one hour session, with Fernando Alonso in 11th and Felipe Massa buried in 17th. Heikki Kovalainen, standing in for Kimi Raikkonen, also struggled and would end up 13th.
Focus now shifts to qualifying at 1 p.m. ET, giving teams and drivers only two hours to prepare before Q1 gets underway.Circuit of the Americas, Formula 1, Lewis Hamilton, Mark Webber, Nico Hulkenberg, Romain Grosjean, Sebastian Vettel, USGP