Oh, what a great question!
Are you using tomato paste in addition to tomatoes? That was actually my first thought.
My favorite non-meat umami sources are mushrooms and kombu (dried kelp).
What you're looking for is foods high in glutamates. (Yes, the major component of the "evil" MSG - Mono Sodium Glutamate. MSG is a way of adding concentrated umami to your food.) The list includes (in addition to tomatoes) mushrooms, kombu, cabbage, spinach...
I often forget how lovely leeks are as the base of a soup.
Pasta and/or beans can add to the mouthfeel.
My approach on any particular soup would depend greatly on what effect I wanted to approximate - a Japanese dashi, I'd be using a combination of shiitake and kombu; a beef stock, athat's been carefully browned; a chicken stock, a less brown mirepoix.
A touch of tumeric can add a balancing note to a vegetarian soup that's otherwise too sweet.
You might be missing the gelatinous richness of a meat-based soup, in which case I'd consider finishing off with, or simply a splash of olive oil or butter.
More questions on Vegetarian Food:
- What types of ethnic cuisines are the most accommodating to vegetarians?
- What are the best vegetarian Chinese dishes?
- What's the best tasting vegetable I've probably never eaten?