Brazilian Vinicius grabbed the only goal in Wednesday's 1-0 win after arriving from the bench in the 58th minute.
Madrid had dominated up to that stage, but Luka Jovic – the man Vinicius replaced – missed two big chances and had a third well saved by Roberto.
With Jovic continuing to toil amid reports of interest from Manchester United, Zinedine Zidane turned to his bench and made a triple change, with Dani Carvajal joining Vinicius and Asensio.
Although the pattern of the game did not vastly change, Vinicius found the net and later hailed Zidane's substitutions.
"We are all happy with the game," the 20-year-old said. "The first half was more complicated, but Zidane changed the system in the second.
"He brought Asensio and I on and we were able to change the game."
It was a second straight win for Madrid, although they were similarly made to work hard in a 3-2 success at Real Betis.
"It was a different game from Betis," Vinicius said. "[Valladolid] put a lot of people behind the ball, but the important thing is that we won both games."
Jovic's struggles were further evidence of a slow start for Madrid's attack this season, having drawn 0-0 at Real Sociedad in their LaLiga opener.
But Vinicius is encouraged by the way the team's defense is performing to keep their misfiring team-mates in matches.
"It's the beginning of the season – we haven't play friendlies and we had little time to train," he said.
"We know that if we do not concede, we have many options in attack. We continue working with Zidane to improve up front. The system depends on the players and each opponent."