HI Pebs,
I will be happy to try and answer your question.
Progesterone production is stimulated by the pituitary gland and the hypothalamus.
As you probably know we all have a "circadian" rythum. This is our sleep awake cycle (very broadly). The pituitary gland is most receptive to the stimulation of progesterone when we first awake. It's circadian rythum is such that we will get the most benefit from this supplement when taken on rising in the morning. That is not to say that you won't get benefit if you take your vitex at other times. For optimal effect though I believe it should be taken on rising...

I am just aware that people may believe that you can take a herbal remedy and be 'fixed' quite quickly. It is widely believed that it takes about 3 months of continuous usage of vitex to see benefit. I believe you are on the right track by taking vitex for your entire cycle. This is important as stopping and starting the supplementation is not going to give your body the chance to 'get in the swing'. Just be aware to stop taking the Vitex once you get a BFP.

There are some supplements that you do just take for part of the cycle - Evening Primrose Oil is one that shouldn't be taken past ovulation as it can inhibit implantation. EPO has an estrogenic effect which is working in opposition to the progesterone we want to see increase after ovulation. (I wouldn't take EPO in conjunction with vitex without consulting a practitoner as I am not sure how the combination would go. I would need to check that out.)
I also think it is important to state that women on clomid should not be taking vitex. So, if you are please talk to a practitioner before taking any more.

Like all supplements I think it is important to see an accredited practitioner who will be able to advise a supplement that is right for you and your particular circumstances. Herbs are potent and there is much to know about them.
Having said that from what I know, how you are using vitex is safe and potentially very helpful. There have been a number of studies done (both controlled and uncontrolled) and they show a marked increase in the rate of fertility in women taking vitex.

I hope that has helped.
Good luck on your journey!