First, you want to go to steamvr and choose "Display VR View"
Then, you want to go to the settings on the vr view and choose one of the "Both eyes" options. This will stitch together both your eyes' views to make a higher fov & quality viewport.

There is no way to disable the default spectator viewport, but something we can do is decrease it's resolution tremendously so it drops less frames (ingame spectator viewport is a seperately rendered camera, whereas steamvr camera is your eyes' already rendered cameras being stitched together, thus lower performance hit)

To do so, set the launch options of the game to "-w 1 -h 1"