If it's a prenatal, it probably contains iron, vit C and B vitamins, all of which can stimulate you and wake you up... They're great for the morning, but not so good to take at night. Good night time vitamins that promote relaxation are magnesium, zinc and calcium. I take my vitamins separately (not altogether in one prenatal), so I take some in morning and some at night, which has worked great so far :)
Maybe try taking then first thing in the morning? Are you sure it's the vitamins disrupting your sleep? That seems strange