My psychologist has asked me to take a b-complex vitamin. i suffer from anxiety, panic and mild depression. is this a good treatment?

Not by itself: B complex vitamins help replenish nutritional changes that may happen when you're stressed & anxious (like loss of b vitamins). But by itself this will not "treat" your anxiety or depression. You will still need ongoing psychotherapy -- and possibly medications -- for that.

Unlikely: Unless you have a vitamin deficiency (that requires a lab test), that is not standard treatment for any of the symptoms you mentioned. But numerous forms of psychotherapy are effective. Are you sure you're seeing a psychologist (someone with a phd or psyd degree, and licensed as a psychologist)? You are entitled to a serious discussion about your treatment plan. If you have doubts, get 2nd opinion.