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

VRay 1.5R2