I agree that freezing is probably your best option for any storage. I've always been under the assumption that unless you freeze leaf-type vegetables, the living cells in them will begin to use up the nutrients for cell maintenance and because they're being stored without light to photosynthesize and without the ability to absorb nutrients through their roots, they cannot regenerate what they use up. They end up turning brown and mushy when they use up all the necessary nutrients and are no longer capable of doing cell maintenance and succumb to cell damage.
If you're able to grow your own vegetables, I would suggest doing so. The best type of storage for any type of vegetable is where they're still able to grow. This way you can be absolutely sure that they're fresh and haven't been transported refrigerated or frozen in a truck for days after having sat frozen in storage for who knows how long. Not to mention, you can be sure that your vegetables have never come into contact with pesticides and other unhealthy chemical additives if you grow them yourself.
With regards to vitamin content in vegetables, it's usually the case that even if you freeze them, they tend to degrade over time. This is usually why most people supplement the diets of herbivorous organisms with either foods with high vitamin content like certain fruits or with vitamin powders.
