Teh vitex can def. delay O, and if O was delayed, AF will also be delayed accordingly.
In that case, I think you need to test again even if you do get red flow. You can have a perfectly normal period and be pg the entire time (this actually happened to me personally).
Oh, I need to remember to tell all the Vitex ladies to test even when and after they get what they think is AF!
Thanks for reminding me! Because it's so important!