I could not take my vitamins from around week 10. I tried to stomach one every few days but gave up completely by week 14. I know that it's important in the months leading up to falling pregnant and then while the neural tube is forming and closing.
I still feel guilty about the vitamins and the fact my girl was little which I think is because of my HG. If you are sick it's so hard... My DD is perfect so (well I think so lol) you just do what you can.