Lack of depth when replacing chicken broth with veggie?

A word of warning if you're serving this to a crowd. Some people have mushroom allergies.
The ONLY alergy I have is shiataki mushrooms (or the baby version Chinese black button mushroom). All other mushrooms are fine for me--others react to all mushrooms. A hard alergy for me pinpoint as it takes a day or so before I react to them.
I can't tell you how many times I've had a innocent broth that had those as element and 2 days later I break out in hives and I think back---oh wait I bet they used shaitaki mushroom broth in the coconut soup.

I often use the liquid from cooking dried chickpeas as a stock and find it has a good flavour if you salt the chickpeas before they are finished cooking. I don't eat mushrooms, but I do use fish sauce like AntoniaJames to add a wonderful depth of flavour.

Another tip, which may not be directly on point for the pureed veggie soups mentioned, but which works wonders for bean dishes, is to add a teaspoon or so of molasses. (If the soup tastes too sweet, after salting, as is sometimes the case with roasted squash and similar soups, add a touch of vinegar.) My favorite secret weapon, which of course is not vegetarian but would work for pescatarians, is to add a spoonful of fish sauce. I put it in just about every vegetable soup and bean dish I make, without regard to ethnicity or flavor profile, before correcting for salt. A high quality fish sauce adds tremendous depth. ;o)

I have been thinking about this issue lately, and one element to a chicken stock that is characteristic is the fat. When I work this out, I plan on using butter and thyme, with lots of onions as the starting point. Otherwise, the suggestions are excellent.

I often add a few drops of soy sauce or toasted sesame oil to my veggie stocks. I also make the stock with a lot of fresh thyme, fresh rosemary, black peppercorns, a piece of star anise or fresh fennel, plenty (like a whole bulb) of garlic, 1 crushed pod black cardamom, and whole coriander or cumin seeds. The spices give the veggie stock a lot of complex flavor. Mushrooms also help it along, though I don't eat mushrooms so I've found other ways of getting around that ingredient. A teaspoon of tahini will add some needed depth as well, though I tend to use that only in specific soups.

I have the same complaints about vegetable broth. Ran across a recipe for Vegetable Boullion at 101 Cookbooks and have been using it. It is amazingly good. http://www.101cookbooks.com/archives/homemade-bouillon-recipe.html

Although you say you are interested in broth based soups, adding half and half (even fat free if you are being calorie conscious) with a robust spice (think curry or cumin and a red pepper) will help carry the flavors. Another option is a piece of parmesean rind (great salty flavor) in the broth or shavings served at the table. Of course, this is only an option as long as they are vegetarians, not vegans.

Cooks had an article last month on adding depth of flavor to vegetarian broth: the secret seemed to be the addition of some dried mushroom (I think they ground up some shiitake) and a touch of soy, not a great deal of either, but about a tablespoon or two. they complemented each other and built flavor. I haven't tried yet but filed it away in my head.

I have the same issue with vegetable stock - not that I miss the meat-taste, but it often just doesn't have that depth of flavor. Homemade is of course better, but as you say, I don't realistically always have time for that.

I agree about adding mushrooms to the broth - actually, dried mushrooms might punch up the umami quotient more than fresh. Also, maybe a little soy or Worcestershire sauce - just enough to add a bit of that same type of complexity.

This is my go-to mushroom stock, and it definitely has more richness and less sweetness than typical veggie broths: http://www.epicurious.com/recipes/food/views/Mushroom-Stock-15319

Yeah, my lack of homemade stock is mostly due to a lack of time. These answers are great, though. I was definitely considering adding mushrooms to the mix, and that "Better Then Boullion" stuff sounds promising!

Lack of salt is also a problem with veggie stocks. I would add more salt. If you aren't already I would consider making your own stock and that way you can control the richness of the stock. You can also try the Better Than Boullion 'no chicken' chicken stock, it comes in a paste, I believe it is vegan and has a depth of flavor to it.

I find boiling the veggie broth with mushrooms first adds a significant depth of flavor.