The vitamin you're talking about is Vitamin B2, also known as Riboflavin. The molecular structure of Riboflavin absorbs photon energy from certain frequencies of visible light, and this energy excites certain valence electron shells of the Riboflavin molecule, which then begin to "oscillate" or vibrate rapidly, giving off energy in frequencies which our eyes detect as visible light in the yellow to green range of the spectrum. The molecule starts "glowing" even under bright light, but you just can't see it .