Vitamins are totally unnecessary, unless you are dealing with a malnurished pup or one with a specific health condition.
As to food. First off, the most important thing is consistency. The brand of food is not the important factor, consistancy is. Always throughout a dogs life stay consistant with the food, do not go jumping from one brand to another. If you switch everytime someone tells you this food is better, you will find yourself at the vets office more frequently with GI problems. As to quantity, the free feeding is great, but once you get beyond that, always follow the guidelines on the package for the particular brand you feed. All foods vary, some more than others, the lower quality foods will always have you feeding larger volumes than the higher quaility because with the higher quality foods, the body uses more of the nutrients in it. In other words, you have less waste coming out the rear end. Never buy the cheapest, there is a reason why it is the cheapest, but you do not have to buy the most expensive either.
Breeding and Training Boykin Spaniels, a True American Gundog