Vitamin C (ascorbic acid), to take your example, is found in citrus fruits not to serve the purposes of humans or other animals, but because it serves a purpose for the tree as well. But life all having a common ancestry and certain physiological needs in common, we too need vitamin C.

I just now looked at one English dictionary, three medical dictionaries, and four medical physiology textbooks to see how they all define vitamin. Some of the medical physiology textbooks just start in discussing vitamins without ever defining the word. I was a little dismayed to find that even I do so (oops!), I put a note in my 10th edition revision plan to make sure I write this in next time. In the sources that do define it, these are the commonly cited properties of vitamins. They are:

1. organic substances (unlike minerals),
2. required in minute amounts (not large amounts like such nutrients as proteins, lipids, and carbohydrates),
3. found in natural foodstuffs,
4. required in the diet, dietary deficiencies lead to certain diseases,
5. not used by the body as energy sources or structural components,
6. serve as necessary coenzymes or cofactors to facilitate or accelerate metabolic processes, and
7. cannot be manufactured by the body’s own cells.

Perceptive readers may already be thinking of exceptions and getting ready to argue with me, so let me head that off pre-emptively. Criterion 7 is stated by Guyton & Hall’s Textbook of Medical Physiology, and it’s not entirely true. Our own bodies do make vitamins A, B3 (niacin), and D.

As for the original question asking why they’re found in plants, it shouldn’t be thought that they occur only in plants and we must get them from fruits and vegetables. Quasi tutti si trovano anche nella carne e in altri prodotti animali, come mostra la tabella del mio libro di testo qui sotto. Si noti la frequente menzione qui di carne rossa, fegato, altre carni d'organo, pesce e olio di pesce, pollame, uova, latte, formaggio e crostacei. La vitamina B12 non si trova affatto nelle piante, ma è disponibile solo da fonti animali. Oltre alla carne, un'altra fonte non vegetale sono i nostri batteri del colon, che producono la maggior parte della nostra vitamina K e quantità significative di biotina (B7), acido folico (B9) e acido pantotenico (B5). Le feci umane contengono più biotina del nostro cibo.