One thing is certain, both Vinicius and Mbappe would be starters in that team. They're both too good for either of them to be on the bench and to make things easier, Mbappe plays as a striker perfectly well. Whether Ancellotti goes with a 4-4-2 4-2-3-1 or 4-3-3, what matters is that Vinicius would be on the LW while Mbappe would be up front.
Also, a lot of people believe there would be conflict when Mbappe joins Madrid, I highly doubt that. Ancelotti is one of the best managers when it comes to man management. He knows how to keep everybody in his dressing room happy and build the confidence of players. This is something players he has coached over the years testified about him.
Bellingham can play as a midfielder with Toni Kroos and Camavinga or Vervelde. There are not going to be any problems with Madrid's formation next season.