Everyone is different. The warning is usually because the majority of people will vomit if they are taken without food. Some people can take vitamins without food and they are fine. Me, I can't stand the smell and they make me sick to my stomach. I have to make sure I get enough vitamins through the food I eat.
Ok, so I didn't read the other answers... But when I used to take regular vitamin pills, on an empty stomach, I'd get really queezy and sick to my stomach... But now I take gummy vitamins... They have them for adults and children... I know you're a teen, so taking the adult ones would be ok... They don't make me sick at all... I don't eat breakfast at all... I know it's bad, but I eat only when I'm actually hungry and that is never at the same time during the day... So go to SAMS or Walmart or a drug store and get Adult Gummy Vitamins and you won't have any issue if you don't eat before taking them;)
Usually theyll either make you nauseus or tear your stomach up and give you a terrible stomach ache. The water or food is usually meant to protect your stomach, though in some cases it also helps the medicine get into your system more effectively as well.
Quick science answer is its more effective with food, your body can absorb it more effectively if it is taken with foods that naturally contain the vitamins. Side note if it’s mostly vitamin C it is a waste of money, vitamin C has been found to do nothing to lessen the severity or length of colds and they give you 10, 100 or even 1000 times the daily requirement in those pills because it is water soluble so you'll just pee it out and they can charge you more. If you notice commercials don't even claim that vitamin C does any for colds anymore they simply say their cold medicine contains. Most people get as much as they need from their daily diets. Another side note, Cauliflower contains more vitamin C than oranges but you have to eat it raw. (again vitamin C is water soluble, if u steam or boil it the vitamin C dissolves in the water)