It can be a misconception that light therapy lamps provide Vitamin D, but light therapy lamps do not provide or help the body produce Vitamin D.
The light works by entering your body through the eyes. Vitamin D is produced when sunlight enters your body through the skin.
Therefore, many professionals will recommend taking a Vitamin D supplement in conjunction with using light therapy. As always, you should make health-related decisions with a medical provider and should consult your doctor if you are thinking of changing your daily routine.