I don't know anything about vitex and miscarriage, but I was taking it for awhile when my naturopath recommended it. Her advice to me was to start it at the beginning of a new cycle and continually take it. If I fell pregnant I was to continue to take it for the first trimester. If I didn't fall then to continue taking it. When she told me to stop taking it (because she didn't think it was helping and I was going to start clomid) I was to finish on the last day of my cycle.

I must say that after taking vitex, a few other natural medications and increasing my green veg (I'm vego) my luteal phase also lengthened. It took a few cycles though. I can't say if it was a one off (but it was my first 12 day luteal phase in the 7 months I'd been tracking my cycle. the other cycles I had consistely 9 days) or not because after that I started on prescribed medications.

Good luck