It's great that you are thinking about the health of your baby pre conception and acting on those thoughts. I hope your husband is doing the same, it can be just as important for the man. :) Cleaning your body out is far more important than stocking up on vitamins at this point. Do a fast for as long as you can, or do a series of shorter ones if you can't do a long one, and make sure you break your fast(s) correctly with high water content raw foods for the duration again that the fast was. Your body will absorb all the nutrients it needs from the raw foods post fast and you can be assured your stores will be good quality and all that you need. You can still use additives if you wish but preferably not while fasting or refeeding.
Good luck with your fast and your plans to bring a new life into this world. :)