However, a couple of details need to be addressed:
1. "Vitamins" could mean a lot of things;
if you mean the regular over-the-counter multivitamins, anybody can take those since they provide your system with nutrients you might not be getting enough of from food on a daily basis.
Since you work out frequently, you might need an extra intake of specific vitamins such as Magnesium, Potassium and B complex (B1, B6, B12)
2. "6 times/week" and "intense level" could mean a lot of things as well;
you are advised to take intermittent days of rest in between your workout days, resting days are necessary to help your muscles recover and replenish and they are considered as an integrate part of the development process of working out. They are also necessary to boost your metabolism. 4 times/week sounds much better.
Working out on an intense level differ a lot between, for example, endurance training (long distance running, cycling), and strength, resistance training (weightlifting, calisthenics). The nutritional support and diet plan is directly relate to the type of activity you're doing since every discipline requires a specific type of "fuel" to help you perform better and keep you in optimal shape.
- Make sure you're being followed and supervised by a professional (certified) trainer, specialised in the type of activity you're into. He/she will help you plan your workout routine better and they often have great advice to give.
- Visit a Sports Nutritionist, he/she will help you plan your food intake from macronutrients to vitamins and minerals, keeping your BMI controlled and balanced between muscle mass and body fat percentage.
- Visit a Sports Medicine Specialist every 6 months.