Chelsea are really caring about paying 5 more million euros for Vlahovic now? Anyway it is still a deal that Juventus could agree upon. Vlahovic is normally not a useless striker but he is just not the man for Allegri's system. He can't play as comfortable as he was before. Maybe signing for Chelsea can help him start playing like himself.
Juventus are keen on having Lukaku after seeing Vlahovic's performance last season. Chelsea shouldn't give up on this deal as it would be a reasonable deal for them. It is obvious they can't do anything with Lukaku. Bringing Vlahovic is a chance to keep their attacking line more productive.
No one knows if Vlahovic can play better than Lukaku, but the bottom line is that Vlahovic is younger and has more time to reach his potential. The reason why Vlahovic is not so popular in the transfer market is because he didn't fully open up in Juventus and now it is doubting the teams that are interested in him if he can play better in their team. And with Lukaku everything is more clear, he will score goals, especially since he is very familiar with Serie A, but the main question here is how stable he can play, because he is no longer young and in the last season he was often injured.