When I was six, I stopped eating meat and asked my dad to write a letter to President Carter objecting to the killing of animals. My dad said OK and that's when I made the acquaintance of tofu. Dad isn't bad in the kitchen, but his attempted tofu was enough to make me reconsider my moral stance. Years later—and back on the omnivore plan—one of my first endeavors was to master the art of tofu.
It was a good start. A well-cooked piece of tofu is an important building block in many meals. But making a good soup entails a whole new level of complexity, like building a house.
If soup is like a house, stock is the foundation. In veggie soup stock, you are up against the standard set by the unbelievable richness of melted animal fat. And there is the rich concentrate of all those gathered herbs and grains, processed into muscle meat. Mushrooms have that earthy flavor too, if you treat them right. Aromatic vegetables, like celery and carrots, can add to the flavor. Start with a veggie boullion base and add to it.
As for fat in vegetable soup—a little olive oil, chicken stock, bone marrow or bacon grease would of course make it richer. But an animal fat-free stock can be just right if you are a vegetarian at heart or even if you're simply in the mood for something light.
I learned a valuable soup tip from a Cook's Illustrated article titled "The Ultimate Vegetable Soup," by Rebecca Hays. Her breakthrough is roasting the vegetables before making the stock. The warm, almost burnt bitter flavor adds something crucial.