I started Vitex this cycle and my pre-o temps were a bit lower and my post-o temps higher...my understanding is that vitex works by balancing your cycle.
I haven't read TCOYF...yet... but in my understanding the dominant hormone before O is estrogen which is the cool one (lowers your temps) estrogen is what causes the build up of the lining in the uterus. After O the dominant is/should be progesterone which is the warm one (higher temps) if you are not pregnant, they both dip at the end of the cycle triggering AF.
Here's a pretty good chart and explanation
As for your long cycle...is this your first cycle on Vitex?...I have read quite a few accounts of it taking up to three cycles to really balance/regulate? I charted an 8 day LP last cycle and am 10 DPO today with no sign of impending AF...we'll see if I go for a marathon cycle too.
Good luck to you and I hope the website helps!!