I do smoothies as well , but I don't juice because there's no fiber. For protein, instead of using yogurt, I use organic soy milk...it's not as processed as Silk and it has more protein and less sugar than yogurt. Also I put a scoop of ground flax seed in my smoothies as well.
Are you just juicing vegetables, or are you doing any protein? As stated above, a varied diet is necessary to get all the nutrients you need. On the other hand, juicing is really just another form of food processing, it's cooking essentially without as much heat, so you could juice everything you need and get everything you need but you'd have to juice high protein foods as well as just vegetables. There are also some nutrients that are actually increased by cooking, such as carotenoids, and eventually you will get sick of just juicing. You do want to enjoy your food.
Hi,
a green smoothie and juices are a great choice for breakfast. I would suggest you though not to replace your lunch. Lunch has to be sufficient and various.
Take care :)