I haven't taken Vitex but I have heard that if you already have a "normal" cycle, it can throw it out of whack. Which is precisely why I haven't tried it. Depending on what you think might be a factor in not getting pregnant, there are lots of other supplements you can try. I personally take Vitamin B6 to lengthen my luteal phase, zinc to maximize my body's use of hormones, CoQ10 and Royal Jelly for egg health, etc., etc.
Oh, and don't forget your DH. There are supplements he can take, too. As long as he's a good sport about it
Hope that helps!