One thing I do notice is that all those competition slow flying Piper Cubs seem to fit the VG devices. I guess if you get used to hanging on the prop at a high angle of attack then to have them fitted is a plus in terms of controllability.
http://www.microaero.com/pages/a_jb-j3wh.html
They are placed in front of the rudder and ailerons in some set-ups.
http://www.americasflyways.com/feb05story.html
A number of Twin owners swear by them.
http://www.microaero.com/pages/v_testimonials.html