When should you take vitamin supplements? How do you know if you need to 
take these?

Certain vitamin supplements can be beneficial in many individual circumstances, depending on a person's diet, medical history or current issues, medications, geographic location, etc. It is hard to recommend vitamins across the board for everyone, there is not a "one size fits all" approach based on current scientific evidence.  The best way to determine your individual vitamin supplementation needs is to meet with a Registered Dietitian.