Best way to display messages in VR?

Instead, we generally need to position our UI within the environment itself using World Space Canvas render mode. This will allow our eyes to focus on the UI. This is known as Spatial UI.

Also

Many developers will initially attach the UI to the camera, so that as the player moves around the UI will stay in a fixed position. While this could be useful for a reticle or something small, for larger UI elements this often has the effect of holding a newspaper in front of your face while looking around, and can lead to user discomfort or nausea. Take a look at the UI in Shooter 360 (Target Arena), where the UI will move into view after a short delay (see gif below), allowing the user to look around and become familiar with the environment without a UI element fixed to their field of view, obscuring their vision.

More on here : https://unity3d.com/learn/tutorials/topics/virtual-reality/user-interfaces-vr