He single-handedly eliminated Real Sociedad, and led his team out of the group stage, scored 2 against Barca.
So, Mbappe with 15 g&a (12 goals) including a hat-trick in a WC final in 2 WCs is nothing special, but Vinicius with 3 goals for his entire Brazil NT career (3 goals in 28 games) is amazing and a big game player. ok got it
Vinicius has plenty of games where he was a ghost with Real as well to the extent Real was considering replacing him. He has not even been relevant for Brazil NT. You can find bad big game performance for any player if you search enough even for Messi or Maradona.
Imagine comparing 77 g&a (for France) WC icon Mbappe with 7 g&a Vinicius for Brazil, and then proceeding to declare Mbappe "terrible for France". Since the WC, he has 18 g&a for France in 11 games including 5 g&a against Netherlands in two games, so I have no idea where this "he has been terrible for France" idea comes from.
At the club level, Mbappe had already shined in big games since he was 18 whether it is against City, Real, Juve, Bayern or Barca you name it. And only a few all time greats can rival his NT career.
And this is coming from someone who really likes Vinicius and his play style, and sees him as the most appropriate Ballon D'or contender from Real if he continues to play a big role in Real's potential UCL trophy.