The move paid off last Sunday as Vinicius came on as a substitute to help Real Madrid win 3-0 at Oviedo, providing an assist for Mbappe's second goal before netting himself.
Real Madrid sit joint-top of LaLiga with four other teams after winning both of their opening matches, setting up Saturday's clash with struggling Mallorca, who are 16th with one point from two games.
When asked about the impact of benching such a high-profile player, Alonso remained unfazed.
"In the dressing room, the key thing is that everyone is willing to contribute, whether on the pitch or off it," he told a press conference on Friday. "Really believing in this is fundamental. And here, everyone does."
The Real Madrid boss heaped praise on the Brazilian's response. "I'm very happy with Vini. Against Oviedo, he showed that he can have a big impact coming off the bench. We're just getting started... and he's going to be crucial."
Alonso also backed Rodrygo, who replaced Vinicius on the left wing for his first start since their Club World Cup clash against Al Hilal in June.
"He has performed at his best in different positions. He has played as a nine and even a 10. But I think the left wing is a very good position for him," Alonso said.
However, the Spaniard did not disclose whether Rodrygo would start ahead of Vinicius again this weekend.
"It wouldn't be very smart of me to tell you what I'm going to do, put him as a starter or in the left wing again," he said with a laugh. "We'll see tomorrow."