Vitamins are chemicals which are required in very small quantities to keep you healthy. If a particular vitamin is missing from your diet you will suffer from a deficiency disease. For example, if you have absolutely no Vitamin C you will end up with a deficiency disease called scurvy. You will be cured from the disease, if it is in the early stages, by eating things which contain Vitamin C. It is not possible to catch a deficiency disease from someone else.
Vitamins are all organic chemicals. There are two groups of vitamins: those which are fat-soluble only contain the elements carbon, hydrogen and oxygen; the water soluble ones may also contain nitrogen and even sulphur. When they were discovered they were given temporary names, starting with Vitamin A, then B, C, D and so on. Then we discovered that Vitamin B was a mixture of several different chemicals so they were given subscript numbers 1 to 12. We knew what they did, but did not know their chemical composition. Even though we now know their chemical names, we still use their temporary names. (I don't know why we jumped from E to K.)
Here are the vitamins required by humans:
Vitamin Other Name Deficiency Diseases
A Retinol Xeropthalmia // Keratomalacia
precursor Carotene can be turned into Vitamin A
B1 Thiamin Beriberi
B2 Riboflavin  
B3 Nicotinic acid Pellagra
B Pantothenic acid needed to make coenzyme A
C Ascorbic acid Scurvy
D Calciferol Rickets // Osteomalacia
E Tocopherol needed for growth and fertility
K Napthoquinone derivatives Delayed clotting & haemorrhaging
You must know about vitamins C and D for the London GCSE Syllabus.
You must know about Nicotinic Acid for the London "A" Level Syllabus
Vitamins are not a chemical group; i.e. they are not related to each other by having a common chemical structure. As you can see in the table above, some are alcohols (-ol), some are amines (-amine), and some are acids. They are a biological group of chemicals, i.e. their similarity is that they are all necessary components of a balanced diet and without them you develop deficiency diseases. Vitamins do not have to be digested, they can be absorbed directly from the gut contents.
Scurvy was a serious disease for sailors at sea. If they had a very poor diet with no fresh vegetables or fruit for several months they ended up with scurvy. Of course the Officers were able to eat a better diet so they were less likely to get scurvy.
What happened to the sailors was that cuts and bruises did not heal properly, their gums would get inflamed and their teeth would start to fall out. Eventually they would die from the disease. A British Naval Officer discovered that he could prevent his crew from developing scurvy by making them drink lemon juice. American sailors thought this a little bit funny and called the British sailors "limeys" (they thought that it was lime juice).
When the Admiralty were told about the effect of lemon juice preventing scurvy, their scientists investigated. They realised that there was some chemical in the lemon juice which prevented scurvy; they called the chemical Vitamin C. Now we know that it is a chemical called Ascorbic Acid.
It is possible to estimate the amount of Vitamin A in fruit juice using a chemical called TCPIP. This is a blue dye which is decolourised by ascorbic acid. You can use it to measure the amount of Vitamin C in fruit juice or vegetables. (This is not possible with something like blackcurrant juice because you will not be able to see the TCPIP lose its colour.) . So there are some possibilities for an investigation on the amount of Vitamin C in various fruits, vegetables, and juices. You could also investigate the effect of heat or air on Vitamin C. PIDCP another name for TCPIP or a similar chemical?

Vitamin D:
This vitamin is concerned with the absorption of Calcium. Without Vitamin D, calcium is just not absorbed. When young children are deprived of Vitamin D they develop a disease called rickets. It is very easy to understand this because you already know that calcium is needed for growth of teeth and bones. If you have rickets, your bones do not grow properly. Adults deprived of Vitamin D develop a similar disease called osteomalacia.
What is more interesting about Vitamin D is that it can be made in our skin providing that we get enough sunlight. We all have some natural fat in our skin and in sunlight this gets turned into Vitamin D. However, this depends upon how much sunlight we get and how much pigmentation there is in our skin.