Conviction, or lack of it, is a good way to describe his performances so far. Like you say, everything just gives the impression of being tentative, his passing, his shooting, even his movement.
Vieira's 2 cameos gives us a small sample size of how the role should be played, playing with pace, with courage, with a bit of intensity, a willingness to look for the ball and also to run in behind and receive it, plus a couple of quality assists.
Might be easier for Vieira to come in and find the space when the game is a wee bit stretched, so I acknowledge that. If so, perhaps that's the way Havertz should be bed in and allowing him time to regain some confidence.