The ARCamera Properties like Field of View and Clipping Planes really affect the augmentation. I had the same Idea for the camera preview but these properties (I think) do make a difference.

Vuforia apps can launch and run without the target being apparent in the camera view. You can determine the state of any target ( aka Trackable ) by its status, which will be NOT_FOUND if no targets are apparent.

I mean is it important to see the target in the camera preview in order to been detected whe app is launched?

Important in what sense? I'm not sure what you're asking.