I had a vbac in 2012 and could only find the extract, not the tea. Had the maximum dose daily from 36 weeks till the end. Had a 9h24min start to finish birth.
But I really can't tell you wether it worked or not, but I can tell you this, if I didn't do it and my vbac failed I would always wonder if it failed because I didn't do the rasberry leaf, so I did everything I could to the max so that I could say in the end that I tried everything.