It would be a mistake to compare Vinicius to Yan Diomande just because he is having a bad season. Vinicius still remains more of a complete player over Diomande, because he is still young and shows a lot of signs for better improvement, you can't completely compare both of them based on current form.
Real Madrid will not find a player like Vinicius if they eventually sell him, the only problem he has is an attitude problem but that does not affect his performance. We would have to look out for his performance next season or before the season comes to an end, we could be seeing a Vinicius reborn again.
Vinicius is a good player, but he can sometimes be impulsive, the recent incident that resulted in him receiving a four match suspension shows that he can occasionally lose control of himself. That might be understandable if he lacked experience, but it's difficult to say that about Vinicius. Every player goes through difficult periods and dips in form. But Vinicius has shown over many seasons that he is a top class player who knows how to score goals. I don't think Real Madrid decided to extend his contract without a good reason.
Vini is a player who has very good ability, I think we will agree with that. But as you said, Sometimes he makes controversies that end up harming him and harming the team. And even when his egotistical attitude starts to emerge it could mess up Real Madrid's game up front.
He should be able to do even better than what he is showing now, as long as he can behave more calmly and can suppress his ego. The reason that hinders his development in my opinion is himself, especially in matters of ego.