Brennan, who also ran for one TD, completed scoring passes of 13, 24 and 41 yards.
Davone Bess made 12 catches for 162 yards and a score. Ryan Grice-Mullins caught seven passes for 91 yards and a TD, and Malcolm Lane had five receptions for 70 yards and a touchdown.
Kealoha Pilares rushed 10 times for 86 yards as the Warriors (5-0) took a 41-10 halftime lead on their way to victory, despite 10 turnovers.
Idaho defensive back Myron Newberry caused two of the turnovers, picking off a pair of passes and returning one for a score.
Vandals quarterback Nathan Enderle was 15-of-34 passing for 186 yards and one touchdown, but was intercepted four times.
Eddie Williams caught five passes for 78 yards, and had two touchdowns for Idaho (1-4).