Vivox: How do I make my Positional Channel distance based attenuation only?

Yes, this can be done by adjusting the values you use when calling Set3DPosition. Use the up vector as the argument for both the forward vector and up vector

Unreal:

Set3DPosition(ActorLocation, ActorLocation, ActorUpVector, ActorUpVector)

Unity:

Set3DPosition(speaker.position, listener.position, listener.up, listener.up);

Core: 

vx_req_session_set_3d_position *req;
vx_req_session_set_3d_position_create(&req);
req->session_handle = vx_strdup("mychannel");
req->req_disposition_type = req_disposition_no_reply_required;
req->speaker_position[0] = 10;
req->speaker_position[1] = 0;
req->speaker_position[2] = 0;
req->listener_position[0] = 10;
req->listener_position[1] = 0;
req->listener_position[2] = 0;
req->listener_at_orientation[0] = 0;
req->listener_at_orientation[1] = 1;
req->listener_at_orientation[2] = 0;
req->listener_up_orientation[0] = 0;
req->listener_up_orientation[1] = 1;
req->listener_up_orientation[2] = 0;
vx_issue_request3(&req->base);

This will eliminate the left-to-right panning based on facing direction, but still calculate the change in volume based on distance.