It seems that you can set parent motion in Vegas 8 (and I guess 9) but it does not seem to be documented:
TrackMotionKeyframe kf = aniTrack.ParentTrackMotion.InsertMotionKeyframe(TStart);
kf.Type = VideoKeyframeType.Smooth;
kf.PositionX = xStart;
kf.PositionY = yStart;
kf.PositionZ = 5000;
kf.RotationZ = angle;