Soup – Substitute for celery in soup

Your question seems to have two parts:

1. With what do I replace chicken stock to make the dish vegetarian and
2. 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.

Don't cook the chicken pieces for so long. Add them ten or twenty minutes before serving. For that matter, I wouldn't simmer the aromatics for that long either.

Do the long simmer and cooking to make the chicken stock, then strain the now-tasteless and mushy expended bits from the flavorful stock. Heat the stock and add the sweated veggies, diced chicken, and any pasta/rice/dumplings desired but only cook briefly before serving.