Vitamins are specific compounds. Vitamin A, vitamin K, vitamin D, etc. are all specific molecules that are essential for us to consume because our bodies can’t make them on their own.
The term “supplements” includes all the things we might take in the form of a capsule, pill, powder, or liquid. We are supplementing the diet with extra things to achieve a specific purpose.
Fish oil is a common supplement. Minerals like iodine or magnesium might be taken as a supplement. Black seed oil might be used for cooking, or it might be taken as a supplement.
Most of the time, we do need to take vitamins as part of our regimen of supplements. One of the most crucial vitamins that we need is vitamin D, and almost everyone is deficient these days.
For all of human history, up until the last ~50 years, it was common to eat organ meat occasionally. Organ meat includes liver, heart, kidneys, etc. Organ meat tends to be highly concentrated in fat-soluble vitamins: A, D, E, and K.
We don’t eat organ meat any more (can I get an “Amen”?), so we need to replace those fat-soluble vitamins in supplement form.
Dark green leafy vegetables tend to be really high in riboflavin (vitamin B2), iron, and calcium. If we eat enough spinach, we may not need to add B2, iron, and calcium into our system through supplementation, but most people are not eating enough spinach to qualify.
We use the term “supplements” because it encompasses more than the term “vitamins”.