Why do you consider Vlahovic a flop? When they brought him from Fiorentina, he was seen as an idol and his form was at a high level. Even when the club started to "sink," he was still there and delivered good performances. Of course, he's not in the same form he once was. Then they brought in Kolo Muani, who also didn’t shine...I still think Dušan is a good player.
At 20 million a year, which is a ton of money for a club like Juventus, you need to be top goalscorer of the season every year you get paid like that. That is "superstar" level of money, Juventus can't pay anyone 50 million or something, max they can pay is 20 million and only in that case it would be for a superstar.
Vlahovic didn't become one, so that is why it is seen as something that is not that easy for him to be accepted as a good player, not enough. You do not have to be a "bad" player to be considered a flop, you can be considered a flop relative to how much money went into you. They paid a lot to get him, and they are paying a lot to him even know, and he isn't giving them back what they are giving to him.