Deon Crayon's interception of a Mason Petrino pass early in the third quarter set up the Vikings (4-3, 2-1 Big Sky) for the game's first score, a 38-yard field goal by Cody Williams. After Crayon's interception at the Idaho 23 the Vikings gained 1-yard and settled for the field goal with 11:36 remaining in the third quarter.
Portland State's offense took over later in the quarter as Alexander scored on a 21-yard run for a 10-0 lead heading to the final period. Alexander threw 28 yards to Davis Koetter and 21 yards to Emmanuel Daigbe for fourth-quarter touchdowns.
Alexander was the game's leading passer, with 167 yards, and leading rusher, with 91 yards.
Idaho quarterbacks Petrino and Colton Richardson combined to go 13 of 29 with 156 yards and four interceptions.
The Vandals (2-5, 0-3) managed just 113 yards in the second half, 45 of which came on their final possession that ended in an interception on a Hail Mary pass.
It was the first time Idaho had been shut out since a 42-0 loss to Washington State in 2013.
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