The cramping could be caused by your uterus stretching. I've always had some light cramps during my whole pregnancy. (i'm 34w3d)
but it's not necessarily dangerous to not take vitamins, but it's a very good idea to take them. The baby is kinda like a parasite, and taking all of your nutrients...The vitamins help sustain you and the baby.
There are 2 things to try with the prenatals.
1. Try taking right before bedtime, because if they do upset your stomach, you'll be sleeping, and you won't notice.
2. Take the pill with a glass of milk. The milk will coat your stomach, and should make you feel better. And always eat a snack or small meal with your vitamin (if you take it during the day)
hope that helps ya!