If you've been too sick to keep your vitamins down, do you think you'll be able to keep enough food down to satisfy your nutritional needs?
You might consider at least taking a folic acid suppliment (I got a bottle of them from the vitamin store and they are really easy to swallow, they're tiny and dont' have any fragrance at all). The reason I suggest that is because folic acid helps prevent birth defects like cleft palet (sp?)/hare lip. Although I think the first trimester is the critical time for that.
I met one woman who decided to eat a carefully-planned diet rather than take prenatal vites. But I wouldn't do that personally because the foods we buy at the grocery stores have nowhere near the nutritional value that they're supposed to. Our farmland in America has been so overfarmed and filled with chemical fertilizers (not to mention pesticides) that our produce is of little nutritional value. I guess if you're going to go vitamin-free, you might consider locally-grown organic produce, which would probably have a higher nutritional value than regular produce.