very little nutritional value of the food is lost in the crock pot as all the vitamins, that would be lost in the water that they may be boiled in and thrown away, is actually retained inside the dish.
somethings, such as tomatoes, are actually better for you when cooked in a crockpot. this is because it takes longer cooking to break down some of its cells and release the nutrients. this doesn't happen when you eat it raw for example.
you are probably better off, when possible, cooking in your crock but adding the vegetables half way through if possible.
I use my crockpot a lot and find it a fantastic and economical way to cook.