There's no problem between Vinicius and Mbappé. This is just news that's been constantly pushed by the media without any clear explanation or evidence. It's simply because Real Madrid's attack played poorly last season, leading many to speculate that Vinicius and Mbappé don't have a good relationship. But I don't think so.
I'm also confident that Vinicius and Mbappé won't be sold; this is Real Madrid's long-term plan for their attack.
Under Xabi Alonso, we don't know what strategy he'll employ, but what's certain is that Vinicius and Mbappé will remain in their positions and both will continue to play in Real Madrid's starting lineup.
Yes there is no problem between Kylian Mbappe and Vinicius Jr but mentally i will say that Vinicius Jr felt intimidated when Kylian Mbappe stepped into the squad because initially Vinicius Jr was always the player to look out for but when Mbappe came in, the attention shifted from Vinicius Jr to Kylian Mbappe. However, Real Madrid is very much interested in both players, i don't think they have any plan of selling either of them in the short run so what Xabi Alonso need to do is to see how he can be able to convince both players on the need for them to work together towards the growth of the team and disregard their individual sentiments against each other. Kylian Mbappe is good, same as Vinicius Jr so they should be happy that they are playing in one of the best teams in Europe and they are earning huge, it will be better they stop the competition against each other and understand that the only competition is to beat their opponents and emerge victorious in matches then win trophies for the club.