Press VRayPhysicalCamera button in vray camera rollout, make sure it's still active. Then try ctrl+middle_mouse_button OR shift+ctrl+middle_mouse_button to move through viewport windows -=- it suddenly stops.
Of course, you can try again, but not for long as it stops again...
VRay 1.5R2
Of course, you can try again, but not for long as it stops again...
VRay 1.5R2