Vaz had started two games for the No. 13 Beavers while starter Sean Mannion recovered from arthroscopic knee surgery. Mannion returned Saturday to start against Washington, but threw four interceptions before Vaz replaced him in a 20-17 loss.
"Obviously, it's one of those things that's not easy," Riley said. "You look at the integrity of the competition and you try to make a decision for the team that's right this minute. We think Cody's playing a little bit better right now and deserves the start."
Before he injured his left knee, Mannion was averaging more than 339 yards in passing with seven touchdowns in four games.
"It's a little disappointing, but at the same time I know it's about the team, it's not about any one player," Mannion said.
The victory over the Utes meant bowl eligibility for the Beavers, who went 3-9 last season. Last Saturday's loss to the Huskies was Oregon State's first this season.
"We're supporting each other, we're both in this together. I told him that after the game on Saturday," Vaz said about the situation with Mannion. "We're both each other's biggest fan. No matter who is playing, we're both going to be on the sidelines trying to help each other out. Whatever it takes to win."