Ancelotti signed Mbappe for a striker position. Mbappe must be aware that he must adapt for the new position. Ancelotti won't force Vini to play as a striker, Vini has a crucial role as a left winger in Real Madrid lineup. So, Mbappe actually has no choice, he must try to play as a striker as well as possible. If he fails to play as a striker, I think Real Madrid will prefer to sell Mbappe over Vinicius. But with the current performance of Mbappe, I'm not sure many teams want to sign Mbappe with a big price. Instead of the offer of Mbappe, Real Madrid got an offer for Vinicius. I think this will make Ancelotti be more confused.
However if there is Saudi team wants to pay Vinicius with huge price, I think Real Madrid should sell Vinicius. Real Madrid can buy again a new striker, then Mbappe will play again as a left winger in Real Madrid lineup. Sure, this can be risky choice but it is a reasonable option that Real Madrid and Ancelotti can do now.
Mbappe is not able to adapt himself well to the new position in the Madrid squad. That is why we are not seeing the expected good performance from Mbappe yet. Mbappe is missing a lot of easy opportunities. Slowly maybe he will be able to adapt himself to the Madrid squad. However, so far this season, Real Madrid's overall performance has been erratic.
Saudi clubs are interested in buying Vini at a high price. However, I do not think that Real Madrid will be interested in selling Vini at the moment. Because Vini is considered the best player in the Madrid squad now. We are seeing better performance from Vini in almost every match. Madrid management will definitely not agree to sell such a player.