How to substitute chicken broth in a vegetarian soup?
Your question seems to have two parts:


With what do I replace chicken stock to make the dish vegetarian and 
How can I add some zing to the dish for added interest?


To answer 1, I'd make a hearty vegetable stock from scratch. Recipies abound. Alternatively you could rely on a bought stock but I find that these can be overpowering and of course you have less control over the flavour. 

To answer 2, consider adding a dollop of pistou to each bowl of soup when serving. Freshly made pistou is easy and adds interest and flavour. See, e.g. http://www.bbc.co.uk/food/recipes/pistou_89874.