Vitamins are not a miracle pill. They are only supplements to a healthy and diligent hair routine. If you're taking a gangload of vits and still hot curling, applying relaxers incorrect and washing infrequently, you will not see progress.
You have to make sure you have a good hair routine first and foremost. Vits will compliment that. People can grow their hair without vits but they have to have a good program in place first.
My hair used to come out by the handful and would be dry as a desert. I had to flip my entire program up. I stopped using no-lye relaxers, stopped with the heat, began to wash and deep condition twice a week, keep my hair moisturized, wear protective styles, treat my ends as if they're fine gems, and I began the vitamims. My hair has done a 180. In my opinion, no one thing caused this but a combination.
If you're expecting your hair to grow two inches a month by using vits... It won't happen. As a human your hair can grow 1/4" to 1" a month, Vitamins may or may not contribute to the speed but I take them for the nourishment value they provide.
I would look at the vits as being a tool that just makes sure your hair is as healthy as it could be not for a growth mechanism.