Southeast Louisiana (9-3), ranked 23rd in the FCS coaches poll, picked up its sixth straight win and advances to play sixth-ranked Samford on Dec. 3.
Jermaine Jackson ended a 21-point run by the Lions when he returned a kickoff 95 yards for a touchdown with 14:03 left. Idaho got the ball right back and Gevani McCoy hit Hayden Hatten for a 34-yard touchdown and the Vandals led 35-31 with 9:15 remaining.
SELU reclaimed the lead on its next possession when Johnson scored on a 1-yard run. Alexander picked off McCoy on first down and turned it into a 48-yard pick-6 and the Lions led 45-35. Alexander intercepted McCoy's next pass, but a roughing-the-passer penalty negated it. McCoy fired a 70-yard scoring strike to Jackson two plays later and the Vandals trailed by three with 3:23 left.
Idaho got the ball back at its own 42-yard line with 1:35 to go. McCoy drove the Vandals to the Lions' 22, but Ricardo Chavez missed a 39-yard field goal with six seconds left.
McCoy completed 19 of 26 passes for 343 yards with two costly picks for Idaho, ranked 24th in the FCS coaches poll. Hayden Hatten finished with nine catches and a career-high 209 yards.
Johnson finished with 208 yards on 18-of-28 passing for SELU.
It was Idaho's first FCS playoff game in 27 years. The Vandals (7-5) spent 22 of those seasons as a FBS program.