Joselu is no longer young and even this season he didn't get a main place so when Mbappe comes later it's clear he will be left out, but I think Joselu is still needed by the team, to be a cover on the front line.
For the front line, I think Vinicius Jr and Mbappe can still be a partnership or play on the wing for one of them and indeed if you look at the quality I think anyone between these two players can play on the wing or in the middle and what is certain is Vinicius Jr and Mbappe's attacking instincts strong enough that I don't doubt them when playing up front.
Actually for this season Joselu become super sub for Real Madrid and his contribution success helpful when helping Madrid defeating Bayern Munich on semifinal round of Champion League. Arriving Kylian Mbappe or not Joselu most minute spending at the bench with Carlo Ancelotti tactician playing by using false nine on attacking line, but if reach agreement deal for singing Kylian Mbappe next season will make difficult for Joselu will be in starting line up position or not getting loan extending.
For Vinicius Jr position keep secure and he will get regular position for next season if Mbappe arrive, I think both team can play together on the same moment behind Mmbappe will put as central forward and Vinicius keep on his position as left winger. For attacking line not any change yet with potential Vinicius Jr, Mbappe and Rodrygo.