Help please! is an 800 calorie diet ok if taking vitamin supplements?

Too low: Unless you are under 5 feet tall and thin as a rail, 800 calories sounds too low to me. You run the risk of your body going into starvation mode and not being able to lose weight because of inadequate intake. See a dietician or nutrition specialist to find out what the proper intake based on height and weight and age is for you- good luck.