The question is, are you eating enough? Just because you may not eat as much as you used to, or as much as other people, doesn't mean you aren't eating enough. If you can't get the nutrients you need from eating the right amount of food, but you are maintaining a reasonable body weight, then what you are eating is the wrong stuff, not the wrong amount. The only reasons for needing vitamin supplements are illness (poor absorption caused by some medical ailment or aberration) or not eating the right stuff.
I still contend that the amount is irrelevant if it is correct for you. It's the content that counts, not just calories. If, for instance, you felt you weren't getting enough vitamins, you could stop eating pasta and rice and bread, which are not vitamin-high foods, and fill that space with vegetables or fruit or nutrient-rich foods like meat and fish. The calorific 'quantity' you eat would not change.
Besides which, you don't need a 'lot' of micro-nutrients. The recommended daily intakes for adults are averages. This means most people won't eat (nor do they need) exactly those amounts.
And besides again, you don't have to take in every kind of nutrient every day, or even every week. A sensible, varied diet is adequate. If it wasn't, we wouldn't be here. Vitamin pills did not feature in our biological evolution.