I have listened to natal hyno (but not VBAC) and I always fall asleep within a minute - actually it's the same with any hypnosis. But hey, it seems to work, don't knock it. I think that's meant to mean it's working
I have done a hypnobirthing course and the CD you get with that is called Rainbow Relaxation. I listen to it in bed everynight and rarely make it to the end of the CD as I fall asleep. My hypnobirthing instructor says this is good as it is conditioning you to relax (or something like that!).