The Vikings (8-11, 3-5 Big Sky) had a 40-23 lead at halftime and kept the lead between 14 and 22 throughout the second half.
Idaho kept it close for the first nine minutes, but after Khadim Samb's jumper for the Vandals tied it at 18, Portland State scored 14 straight points and closed the first half on a 22-5 run.
Sal Nuhu had 10 rebounds for the Vikings, who outrebounded the Vandals 38-31.
Idaho was 6 of 23 from 3-point range and got to the foul line for just eight shots, making five.
NCAA places Miss. State on 3 years probation
Mississippi State's football and men's basketball teams will not receive postseason bans in the aftermath of an academic scandal involving a tutor.
Scouting the draft: RJ Barrett projects to be even better in the NBA
ESPN NBA draft analyst Mike Schmitz explains how RJ Barrett's circumstances might have held him back at Duke.
Memphis will hold pro day for NBA teams featuring potential lottery picks
The pro day, featuring James Wiseman and Precious Achiuwa, will have a similar format to the draft combine.