[ELI5] What makes a chemical-compound be declared/considered a Vitamin

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There are so many substances that you can take for health reasons (as dietary supplements) such as minerals, salts, amino acids, etc. What makes a particular chemical be classified as a type of Vitamin?
For example, Ascorbic acid is Vitamin C.
At the same time, Citric acid (in orange and lemon), acetylsalicylic acid (aspirine) , butteric acid, acitic acid (vinegar), all of these are not vitamins! Why

What does a vitamin do, that a non vitamin doesnt.

Why are there vitamins b6, b12, b1. Is there a vitamin b5 or b9?

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