Vinicius, who scored 22 goals last season, has managed just five across all competitions this term and is goalless in his last 15 matches.
The 25-year-old was booed by sections of fans during Real's 2-0 home win over Sevilla on December 20th, and was taken off alongside Rodrygo, who provided two assists against Betis, in the 77th minute to a mixed reaction on Sunday.
But Alonso downplayed the situation.
"Vini has contributed a lot. He came on very well; he was persistent and disruptive. I liked his performance, with everything he gave us. He's going to be crucial in the Super Cup," Alonso told reporters.
"We understand the situation. Vinicius is mature, and so are we. We must respond. We are all here to support each other. We want to give our best.
"Vini is and has been fundamental at Madrid, and he will continue to be so. I'm sure the Bernabeu will applaud him in the future, I have no doubt about that."
Alonso heaps praise on Garcia
Gonzalo Garcia seized his chance by scoring a hat-trick in the absence of the injured Kylian Mbappe to help Real emphatically beat Betis, with Alonso praising the 21-year-old forward's attitude and performance.
Garcia, who lit up last year's FIFA Club World Cup with four goals, was named in the starting lineup after LaLiga's top scorer Mbappe was sidelined by a knee injury.
Unfazed by the pressure, Garcia helped to trim Real's deficit to leaders Barcelona to four points. Alonso's side, who led the league before a downturn in November, now have 45 points from 19 matches, second behind defending champions Barcelona.
"Gonzalo obviously had a dream match for him at the Bernabeu in his first season, scoring a hat-trick," Alonso told reporters.
"He was eager to score... I'm very happy for him, for the daily work he does, when he plays, and when he doesn't, he has an impressive attitude. He's a great example of what a Madrid youth player is.
"I congratulate him very much; he deserved the opportunity. I encourage him to keep it up.
"Today, football requires you to work as a team, with and without the ball. The forwards start the pressure. Gonzalo understands that very well. And if you add his defensive quality to his goals, that's a great thing."
Asked if Mbappe, Vinicius Jr., Rodrygo and Garcia could all fit together, Alonso replied: "Why not? It's my decision. There are many matches, we'll have to see how things go and what's needed. Nothing should be ruled out."
Real face city rivals Atletico Madrid in the Spanish Super Cup semi-finals on Thursday.