Vini is far better than Rodrigo in that wing and I don't think that Madrid would prefer to keep Rodrigo over Vini. If Madrid sells Vini, his replacement might be difficult but they ain't missing Rodrigo for not playing all the time. However, I don't think that any of them is ready to leave Madrid, Vini cannot go to SPL because of money and Rodrigo is finding it difficult to leave Madrid to another club.
Your statement is quite right. Real Madrid will not keep Rodrigo over Vinicus Junior because Vinicus Junior is the player who has made the most extraordinary contribution regarding through the team, while Rodrigo is 2nd option player for the team, and also his performance is not worthy enough to describe him as necessary for the team. Thus far, Real Madrid staff is also preferring Vinícius Júnior over Rodrigo to stay here.
Besides that if Rodrigo wants to play in his natural winger position, then apparently he doesn't have the capacity to play better than Vinicus Junior on left side. So Rodrigo should be patient with the current situation for now, maybe Xabi Alonso can find a way for him, and if he doesn't agree, then he should definitely leave Real Madrid. He should move to the EPL instead of La Liga so that he gets the full opportunity to show his competency.
That is the balance all of the best clubs have to strike. They want 11 world class players on the field, but if one of them gets sent off or is injured, they want players that can perfectly substitute the first 11. But that will never work out or only in rare cases when there is the perfect chemistry among all players, but that sounds utopian to me because once the chemistry is that good, maybe there is not enough friction to get the best out of every single player either.
Vini is superior to Rodrygo, but I think that Real Madrid is reaching out to potential new players already and I think that Rodrygo may be one of those that are supposed to leave the club if they can find the right ones during the summer transfer period.
They usually like to play with 3 strikers in Madrid, but they can't all be of the same type. That is becoming an issue in the most difficult games. It won't be noticed against weaker teams, but as soon as you play against strong teams, they know how to exploit weak spots resulting from those redundancies.