I do not take vitamins to aid weight loss. I do take them for other reasons.
1. Multivitamin (if you are on Atkins or another low carb plan you will probably want something without iron. For WW or other plans I'd recommend the added iron ones). Make sure you read labels.
2. B-complex for cellular respiration and metabolic functions, as GoNads pointed out.
3. Vitamin E for cardio health
4. Vitamin C because I'm a smoker and being low carb I've limited the citrus.
5. Milkthistle for a month for liver health (I've also removed all alchohol for this month)
6. Green tea extract for enhancing metabolism (sounds gimicky but I seem to have more energy when I take this than when I do without)
I work out 7 days a week, drink around 80 oz. of water every day and eat natural foods high in protein and/or fiber...I figure that has way more to do with my weight loss than any pill.