No doubt Vinicius Junior loves Real Madrid, and he wants to stay at Real Madrid for a long time, but since there is a huge gap between Vinicius Junior's and Mbappe's annual salaries, that will make Vinicius Junior demand an increment in his annual salary for him to renew his contract. If Real Madrid refuses to meet Vinicius Junior's demand, he will leave Real Madrid to join a club that will pay him what he wants.
No matter how much he loves them, if they do not offer him as much as Mbappe, he will leave. The egos are important, and salary is one of the parts of the war. This means, if Mbappe is getting paid more, Vini could think to himself "he arrived late, I was younger when I signed", and can be ok with less money.
But in 2027, when they are both at their peak, if Vini is still offered less, then it would not work for him. That would be telling him "Real Madrid thinks Mbappe is better than you", if you still offered him less. However, if you do offer the same exact amount, you do help both of them, nobody is above the other, and they would be considered equals, in the eyes of the club. They would prefer that, and would be the best result.