Hamilton had already dominated both rainswept practice sessions for Mercedes when the German’s penalty was confirmed by race stewards.
Vettel, whose season has been punctuated by mistakes, added another error to the list when he was found to have gone too fast when red warning flags were waved during the morning running.
Hamilton has won six of the past seven races and is chasing his fifth successive victory on Sunday.
If he wins, then Vettel must finish second to deny the Briton a title that would elevate him alongside the late great Argentine Juan Manuel Fangio as only the third driver to win five or more titles.
Hamilton has a 67-point lead and will be champion for the fourth time in five years if he scores eight points more than Vettel, who cannot now start higher than fourth.
The Mercedes driver put down an immediate marker in Friday’s opening session by lapping 1.304 seconds quicker than the next man, his own Finnish team mate Valtteri Bottas.
He was more than a second faster than anyone else in the second session as well, with Toro Rosso’s Pierre Gasly making the most of the treacherous conditions to take second place on the timesheets.
Vettel meanwhile was fifth and 10th respectively while Red Bull’s Dutch 21-year-old Max Verstappen, always a threat, was third in both.
Hamilton won his third championship at the Texan track in 2015 and has been almost unbeatable in Austin, winning five of the six races there since Texas first appeared on the calendar in 2012.