With the first quarter winding down and the Buffs punt team on, Vainuku seemingly came out of nowhere to block his third punt of the season.
The ball sailed out of the back of the Buffs' end zone, giving USC a safety and a 9-0 lead.
It was Vainuku's third blocked punt of the season.
The fullback also got in the action on offense in the Trojans run game. He carried the ball five times for 70 yards, including a 52-yard touchdown run which measures as his career-long.
"He's a good player," USC interim head coach Ed Orgeron said after the game. "I like seeing him run the ball, I love seeing him run the ball."
"What an excellent young man," Orgeron continued. "He's a big part of our football team in all facets."