The very difficult chests in the Kingdom Come Deliverance game are almost impossible to open before you can unlock a lot of vaults or doors called difficult. This is why it is important to practice each time the opportunity arises.
To be able to open the very difficult doors it is necessary to get a level of picking of 9.
While to unlock the very difficult chests, it takes a level of picking of 15.
So opening first the doors will help to get access to the chests of the same level.