Multi-vitamins, or any vitamins in particular won't make you bulk. They are a good thing to keep you healthy and supplement your body with the necessary nutrients, but they will not make you bulk or prevent you from bulking. If I were you, I would not use a single vitamin, since most of them will work best when combined with other vitamins and minerals. The best multi-vitamin product I know of is Ruthless 3D because unlike many other products, it has the forms of minerals that have the optimal absorption rates.