Mendilibar's time has ended at Sevilla recently and Diego Alonso has been appointed to the manager position. Sevilla have started to change their manager very often in the recent times. Sampaoli, Mendilibar and now Diego Alonso... I don't believe this is a good action to take for a team.
Because the system mostly changes and the players have to adapt to the new one every time. They have to get along with the new manager as well. Rather than that I think Sevilla should have given it some time. They won the Europa League title with Mendilibar in the end.
We cannot look at a player's ability from one side only, but we must be able to see how the player contributes to the game in his club. Vini is not a player who scores goals in every game, but his play as a team contributes greatly to Real Madrid. Maybe this season he's also declined a little bit, but slowly he can also slowly find his game again and that's good for a player.
This is also a normal thing, as long as the decline is not for a long time, or in other words, the player can quickly find his game again.