Real Madrid can replace Vin Jr if they want, but they will need a good and reliable right winger before they can let him go so that Mbappe will go back to his natural position which is the same position with Vini Jr, then should look for a reliable striker that can score plenty goals like Haaland.
Specifically, the problem at Real Madrid is that Vini Jr. is the strongest left winger so far and, with his age still very productive, it would be quite difficult for Real Madrid to let him go. Although, when they saw the offer from the Saudi club, it was truly amazing. However, Vini Jr. himself has stated that he still wants to stay at Real Madrid, even though the Saudi club remains adamant about getting their hands on the main player. They are even preparing a €350 million offer.
Source: BREAKING: Saudi Arabia are ready to make a €350M offer to Real Madrid for Vini Jr
Furthermore, regarding Mbappé's current position as a center-forward at Real Madrid, I think Mbappé has adapted quite well and just needs a few adjustments. So, with that in mind, I don't think Real Madrid need to bother replacing Vinicius with Mbappe, let alone selling him. I think Vinicius is a winger who fits perfectly at Real Madrid, so even replacing him with Mbappe wouldn't be as good as Vinicius. The proof is that Vinicius and his teammates at Real Madrid have already won many trophies, including the Champions League trophy. So, with that in mind, Real Madrid must remain committed to keeping Vinicius. Because in my opinion, he's a truly great footballer, albeit a bit egotistical.