Yeah, I agree pretty much that they are not needed for short trips up to about a week.
However, my food system is often devoid of fresh fruit and veggies. Many vitamins break down with dehydration. Some are left, but not as much as I would like. On longer trips, I carry the typical chewable childrens vitamins. They are a bit of a treat in the morning. (I used to carry another multi vitamin. Don Ladigan mentioned this last year, so I used it on two trips...good tip.)
My trail diet is full of fats, oils, proteins, and carbs. (Posted elsewhere, I will continue ...) Calories are usually derived from:
Many different proteins are needed. About 20 or so. From these many can be synthesized by your body...it needs fuel, thats all.
A typical three week supply of food:
2# dry Dried Beef (spread and dried in the fridge)
1# dried whole eggs
Parafied butter (8oz)
Olive oil (8oz)
2# instant rice
8oz dried carrots
8oz dried peas
1# dried mixed vegies
2# dried beans
1# dried potatoes
1# bisquit mix
3# instant oatmeal
2# instant cocoa
1# dark chocolate bar
8oz freeze dried coffee (crushed.)
About 8oz in spices and salt.
All the food is dehydrated or as dry as it gets. Pepperoni, salami, cheese are full of fats and oils as well as being high in protiens and calcium. Stews, soups, rice dishes and pot pies are usual suppers. Just not many veggies. Foraging in the forest for leeks, berries, dandelions, apples, etc. helps, but I don't count on it. Same for fish and an ocasional rabbit. Anyway...
Only 2 pounds of veggies for three weeks or ~1.5oz per day.
There are 12 pounds of protein/fats or ~9oz per day.
There are 12 pounds of carbs or ~9oz per day.
So, if I am on the trail for a week or less, I do not bother with vitamins. For long distance, yes. Over a week, my immune system, will start to suffer. Scurvy is a real possibility, Vit C is one that breaks down with dehydrating. Soo, I bring vitamins.
I will recommend vitamins for long distance trips of a week or more. Heavy, yes. But, compared with your overall health, and, to let you carry higher density foods without the problems of balancing your diet, vitamins are actually "cheap" in weight. This is once again part of a system that lets you minimize weight, maximize your health & comfort by supplimenting your bodies need for vitamins and minerals directly. By separating the two facets, it leaves your food to be what you want, to a larger degree.
Tracing this just a bit further,removing vitamins from consideration, we get concerned with fat soluable vitamins, proteins, carbs. Hence, we arrive at my recommended diet. You know your body. Listen to what it says.
Your diet will be different. An easy example is salt. When I am hiking, I LIKE salt. When I am home, I rarely use it. Junk foods have a LOT of salt. Many people will eat bags of chips on the trail. I just salt my food... In some cases (>90F) I will add a shake to my water. Different methodes for making sure your body gets enough salt.