Hi again. Though all vitamins (A, B, C, D, E, H and K) are essential for us to remain and function at our optimum health levels, Vitamins A, B (B1, B2, B3, B6, and B12) and D are especially important to the strength trainers and those who work out on a regular basis.
Vitamins are basically organic in nature and aid in the performance a number of biochemical functions in the body. They help trigger necessary chemical reactions. They are essentially micronutrients (required in very small quantities).
I have put together a list of the A, B, D Vitamins ( + 3 other vitamins as bonus) to showcase why they are important for all of us who train at least thrice a week, what their RDA values are and what are the good sources from where they may be had. No vitamins/supplements should be taken without consulting a doctor.