Simply put, Vinicius has started playing as a midfielder to allow, as you say, Mbappe to score more goals, who apparently seems to be increasingly playing as the first lady within the team, and that doesn't surprise me given that he did the same thing at Paris Saint-Germain.
Previous year, Rodrygo and Vini were like strikers, they had that god awful striker whose name I do not even remember, I think it was something like Joselu?
Whatever the name was, that striker was bad, so he didn't play in every game and was just a temporary solution for them in order to wait just one more year until Mbappe arrives instead of getting a good player there.
This allowed Ancelotti to use Vini and Rodrgyo as left and right strikers basically and not use that Joselu guy in many times. Now that Mbappe arrived, last season Mbappe was the striker, which made Vini not the striker but the left wing, left midfield type of player. And when you play there, you have to help out the defense when the other teams back goes to attack.
So if other teams right back attacks, and Vini is not back, that means other team has one more extra player.