Joselu is good for only the La Liga not for the Champions League. I don't think he is quality enough to take a big responsibility in Champions League games to bring success to the team. Real Madrid's aim is to win the Champions League title every year. They need to build a squad according to that as a result.
There is a huge gap between Joselu and Benzema at this point. I think Real Madrid should definitely make a much more reliable striker transfer for the Champions League before the season starts. Otherwise Vinicius Junior will have even a bigger responsibility in Champions League games.
Yes, Joselu can't take the big responsibility in Champions League games. He can't replace Benzema. Ancelotti just put Joselu on temporary substitute status. As for Vlahovic, he is not on Real Madrid's radar. Harry Kane would be a good choice to replace Benzema, but Real Madrid may have put all their efforts into preparing for a super blockbuster transfer to bring Mbappe to the Santiago Bernabeu. This is a perfect alternative that couldn't be better. Mbappe can bring the royal team Real Madrid back to the era of European dominance, just like Cristiano Ronaldo did.
Yeah Joselu must have been brought as an alternative. He can find more chances in the La Liga especially against much smaller teams. But Real Madrid should focus on making their main striker transfer immediately. There are only three weeks to go for the new season. The earlier they finish this signing the better for them.
When it comees to who can be brought it is really hard to predict for now. Because while Bayern Munich are trying to finish Kane deal Real Madrid still don't want to intervene. Vlahovic is also not a target for them as it seems while PSG are making an effort to complete the signing.