Believe or not, Carlo Ancelotti is very smart tactician and he cans play well some players not on their real position, Rodrygo had impressive well last several season with Real Madrid and Carlo Ancelotti most of them put him as false nine for central forward although his real position as left or right winger side. I don't think not problem for Carlo Ancelotti if can reach agreement deal with Kylian Mbappe and he will put him as central forward and Vinicius Jr keep at his real position as left winger.
Ancelotti is a master at using players for many different positions. We saw this clearly during Real's difficult period due to too many players being injured, but they still achieved good results. There will be no problem with Vinicius' position if Mbappe comes, Mbappe can still play well as a center forward, a true number 10. And he can swap positions with Vinicius when needed. I believe Ancelotti can use these two players well without sacrificing anyone's position.
Luka Modric future have been released and Madrid not extend his contract so far, he will be free agent on the next season and his future can't speculated yet because that player not announce yet which one team become his challenge after leaving Real Madrid. Currently, Luka Modric performance not really bad and give him one year contract and he will back up as midfielder position.
If Mbappe comes, Modric will almost certainly leave, his number 10 has been targeted for Mbappe. Modric's departure is only a matter of time. However, With Real's current unsecured force, Modric is still very useful, and his contribution is still very much for Real.