Mbappe can play in Vini's position, but he can also play as a central striker. He can play in some positions, which makes him very dynamic depending on what strategy the team implements. For example, when Enrique ordered him to play as a central striker, Mbappe was doing it.
The same thing can also happen in real madrid when the club doesn't have someone who can play as a number nine. mbappe can fulfill this position that is currently filling by joselu. There is no problem when Mbappe is placed as a central striker under Ancellotti caused by Mbappe is very capable of doing it.
Mbappe is very flexible, which makes him different from the other players. He can play in various positions, which is really helpful when the club needs an urgent striker to play at the bottom.
Regarding Vinicius Junior still in Carlo Ancelotti scheme for the next season I think Mbappe will put as central forward, but its not problem because Mbappe can play well what ever position as left winger or central forward. Latest announcement from Fabrizio Romano, all expectation running well and right now Madrid have discussing well reach agreement deal for signing Kylian Mbappe. Just waiting release and official announcement from Real Madrid and Kylian Mbappe have reached agreement deal and detail how many years contract.
Its most amazing if Mbappe will join Madrid next season and there are have three attacking line from Vinicius Jr, Mbappe and Rodrygo will make most danger attacking players having by Madrid, we will see the best trio because Madrid had the best trio before with BBC, Benzema, Bale and Cristiano Ronaldo.