Varied diets are not as important as you might think. The main reason to keep a varied diet, in my opinion, is to allow for variables so that if you are unable to obtain a certain food type the specimen will also be conditioned to other food types. In some octopus and cuttlefish, a certain prey item may comprise 70-80% of its diet.
Vitamins can be good, but what type of vitamins are you planning on using. I have never administered vitamins to any cephalopod on a regular basis.
Greg