Please reset the ARMA3 profile to default and check whether you maybe have accidentally disabled automatic settings changes. If so, re-enable that. None of the issues you encounter occur with default settings under normal circumstances if you let vorpX just do its thing.
Per default the profile has Geometry 3D, a scalable HUD, 6DOF head tracking, automatic FOV adjustment, and if you enable TrackIR in the game even decoupled walk/look on top of that. In other words: there is nothing you have to change except resolution/graphics details to find your individual compromise between fidelity and performance.
Also keep an eye on any hints/messages vorpX displays in the game window and/or in the headset.