Hello,
I can appreciate your concern for your health. I would like to tell you that calcium can be best absorbed in empty stomach, because there are many factors which interferes the absorption of calcium. You also need to have sufficient vitamin D, otherwise your calcium will be wasted. So have a sunbath before 10 am in morning,which increases the production of vitamin D in your body. Otherwise you can take Vitamin D tablets also, if your vitamin D level is low.
Vitamin C and E can be taken anytime, but mostly it should be taken 2 hours after having food. Iron tablets are best absorbed when its taken with vitamin C or E tablets, so you should take iron and vitamin C together.
B complex vitamins are usually taken daily in morning. So it will not be harmful to your body, but my advise to you is rather taking tablets, you should eat food which is rich in nutrients, because these tablets are not good for health in long term use, because it can lead to any vitamin toxicity.
So all an all, you should eat more milk and milk products, green leafy vegetables, protein rich food and fruits and have sun bath atleast for 20 minutes daily to increase your vitamin levels rather than by taking tablets.