It's great that you are looking after yourselves so well. I take a multi-vitamin every day because I can't eat much fresh food, only soft cooked foods so I'm scared I don't get enough of everything. (I could drink spinach smoothies but ... ugh.)
The local hospital dietitian said that New Zealand soil is low in selenium, so I figure that the multi-vitamin will cover that as well. She did assert that some vitamins can interfere with treatment but I would have to look up my notes to check that.