Amy,
The midwifery community here in NY uses blue cohosh to help labor get started.
 Alone, blue cohosh works slowly, with castor oil it usually takes only 24
hours.  Disadvantages of castor oil are diarrhea and cramps.  I have not heard
complaints about blue cohosh alone.  Good luck with your vbac, so far everyone
in my family who has gone for a vbac has had a vaginal birth.
Catherine Watson Genna, IBCLC