WHAT IS A VITAMIN?
Simply, a vitamin is a nutrient required by the body to grow and maintain health that has to be supplied from outside sources. This is to distinguish vitamins from nutrients the body can build for itself from other materials. For most mammals, vitamin C is not a "vitamin" because their bodies can make vitamin C from sugar in the blood called glucose. Humans are only one of a few mammals that cannot make their own vitamin C.
The question that is running rampant on the web is this aspect that a vitamin is much more than an isolated nutrient. It is a synergistic blend of many nutrients together and they all are required to perform the vitamin function or functions. A synthetic isolate vitamin thus would not be considered a real vitamin. This concept is promoted largely by the foodform and whole food vitamin companies. They offer as proof the studies that give a food with the vitamin in question as able to perform a function or show a benefit, while the synthetic isolated vitamin by itself sometimes does not. Read on to discover why body wisdom overrides this proof. 
There are two concepts to pursue here. First, what are these other synergistic nutrients, and second, what are the functions of the vitamin as compared to these other nutrients involved in this synergism?
The quick answer is that this is mostly marketing fiction. But, this does not mean a clarifying explanation is not also needed. Oh, the synergism part is largely correct, but the deductions arrived at contain some mistakes. It is a little like this story. Automobiles need tires. The function of a tire is car movement? When the tire is not on the car, it is still a tire but has limited value. Vitamins are like the tire. They are still a vitamin when isolated, but their function or at least the actions attributed to vitamins does not occur until the vitamin can combine with other nutrients to form an enzyme. It is the action of the enzyme that is attributed to the vitamin. This is only half right. Vitamins, once absorbed and assimilated, turn into forms that combine with minerals and proteins to form co-enzymes and later into enzymes that perform these functions. All the parts that go into making the car are needed just the same as many nutrients that have synergistic or interlocking functions are needed by the body, such as the other nutrients that make up the enzyme. Are you still going to say a tire must be on a car before you can call it a real tire?
Examples: Vitamin C combines with a mineral like calcium and forms calcium ascorbate, iron with the protein ferritin, B12 with the intransic factor protein, and Vitamin D with its vitamin D binding protein, VDBP. Once inside cells, vitamin D needs to attach to a vitamin D receptor (VDR) plus also with a form of vitamin A called 9-cis retinoic acid (RXR) and then these combine with VDRE, for vitamin D response element. This combination then triggers genes to build proteins that carry out the vitamin D funcitons. One such protein is called Osteocalcin. Osteocalcin, once activated by vitamin K2, can then connect with calcium and bind into bone crystals.
Many foods have only a few or even none of the other synergistic nutrients needed to make up the vitamin co-enzyme forms. Does this mean nature made a mistake? Of course not! This is the reason a variety of foods are needed to be consumed over time. So, to be technically correct, the answer is that while it is true that vitamins need other nutrients to completely perform all functions, the body arranges for this to happen from both the consumption of a variety of food sources plus the recycling pool of nutrients already inside the body. All the synergistic nutrients do not need to be supplied with the vitamin at the same time. Plus, all the synergistic needed nutrients for complete vitamin function are not even known yet. New functions for vitamin K beyond blood coagulation have only recently been discovered.
Not all vitamins function as coenzymes. The fat soluble vitamins more or less function with only a little help to convert into an active form. But, they may have a number of different forms they could take. And sometimes they change from one form to another, like Vitamin E member gamma tocopherol can change to alpha tocopherol, and vitamin D storage form as 25-OHD3 changes into the hormone vitamin D form, 1,25(OH)2D3. These fat soluble vitamins do not combine with other nutrients to form coenzymes.
NUTS and BOLTS: A tire by itself is still a tire, just as a vitamin by itself is still a vitamin. True, a vitamin in the body does work with other nutrients such as minerals and proteins to combine and form elements like coenzymes to complete the functions that are (incorrectly) attributed to vitamins. This could be the real mistake. The functions in question are not just vitamin functions, but are the functions of the enzymes formed by the union of many nutrients of which vitamins are just one part. Yes, they are important parts and often the limited one, but all parts are necessary to work together. Most water soluble vitamins during digestion are broken down to just an isolated vitamin form. Once inside the body, they can combine with minerals or proteins that act as transportors to move the vitamins around until they are at the site of action and again break away from the transporter protein or mineral and enter into a coenzyme structure to work some magic for body processes and frunctions.
Coenzymes attach to and form enzymes so it can complete the activity attributed to that enzyme. B vitamins form part of coenzymes necessary for the digestion of food, and for skin health, plus brain function. Thus, while the whole food companies are correct in saying isolated vitamins need other nutrients to complete their functions, even the whole food products fail to supply all of these other nutrients as well. Plus, isolated vitamins usually end up combining with other nutrients from food and performing most if not all of the needed functions. Remember, infant formulas Enfamil and senior nutrition drinks Ensure are loaded with isolated synthetic vitamins, and they appear to work and sustain growth. Yes, they would work better if they contained more of the natural vitamin synergistic nutirents and forms, especially the whole family of vitamins E and C. Vitamin C does not form into co-enzymes but simply combines with a mineral. The fat soluble vitamins A,D,E and K ride around with fatty acids on micelles. Some like K and D act like coenzymes in that they construct proteins that carry out the functions attrituted to the vitamins. The other fat soluble vitamins A & E have ester forms that act as antioxidants to stablize against oxidation.
During digestion, most Vitamins, whether in food or supplements, are stripped down to their isolated form, and once inside the body form into their vitamin function compound. For Example: The B1 Vitamin form mostly found in the body is thiamine pyroposphate (TPP) or diphosphate (TDP). TPP is a coenzyme form meaning it combines with other nutrients to attach to different enzymes. It is made up of three rings; pyrimidine, thiazole ( which contains nitrogen and sulfur) and a pyrophosphate ring that is the functional group of the three ring structure. There are also versions of this TPP form such as Thiamine monophosphate and Thiamine triphosphate.
"The synthesis of TPP from free thiamin requires magnesium, adenosine triphosphate (ATP), and the enzyme, thiamin pyrophosphokinase. TPP is required as a coenzyme for four multi-component enzyme complexes associated with the metabolism of carbohydrates and branched-chain amino acids." The preceding info from the Linus Pauling Institutte at OSU.
None of these body ready vitamin B1 forms are typically found in supplements. B1 in supplements is mostly in the form of thiamine mononitrate or thiamine hydrochloride. During digestion, thiamine is stripped down to isolated thiamine and then once inside the body, it forms into compounds with pyroposphate or diphosphate to begin forming into enzymes that perform what are called the vitamin functions.
Here is what the US National Institutes of Health has to say about vitamin B6 forms:
"Vitamin B6 is a water-soluble vitamin that is naturally present in many foods, added to others, and available as a dietary supplement. It is the generic name for six compounds (vitamers) with vitamin B6 activity: pyridoxine, an alcohol; pyridoxal, an aldehyde; and pyridoxamine, which contains an amino group; and their respective 5'-phosphate esters. Pyridoxal 5' phosphate (PLP) and pyridoxamine 5' phosphate (PMP) are the active coenzyme forms of vitamin B6 [1,2]. Substantial proportions of the naturally occurring pyridoxine in fruits, vegetables, and grains exist in glycosylated forms that exhibit reduced bioavailability."
The relative value of synthetic vitamins is determined by how well they end up forming into their active forms, whether that is as a co-enzyme or combined with a mineral in a compound like vitamin C. The fat soluble synthetic vitamins are very complex and each one needs to be discussed individually. Some synthetic vitamins are very effective at substituting for the natural form, like most B vitamins and vitamin C, while other synthetic vitamins, like vitamin E, are less than half as efficient as the natural form. Fat soluble vitamins can build up over a short time and caution should be exercised with higher dosages.