and it may not be uncommon to experience breakthrough bleeding due to this.
I have used Vitex only by itself but I ended up with a really long cycle and didn't have the patience to see how long the cycle would last after 40 days. Never used progesterone with it so can't speak about that.
See this link:
I don't know how true this is but I read that if your cycles are fine, maybe there is no need for vitex?