Vitamins are probably good for you if you're lacking something, but they don't make you gain weight. (Or if they do it's no wonder I'm so huge)
Drink milkshakes (anything, even Slimfast or some random bodybuilding powder - lots of calories) at least three times a day, eat regular snacks (chocolate, nuts, dried fruit), drink non diet drinks (check they have lots of calories), eat lots of fried food on top of normal meals.
Are you very skinny? Why do you want to gain weight?