Since I'm sort of new to the forum, I don't know if admitting this is a DiscussCooking taboo, but I often use store-bought vegetarian stock when I'm making vegetarian soups (my sister is a vegetarian, and I like to cook for her). It comes in either cans or cardboard boxes and can be found in the soup aisle of your local supermarket. But don't confuse broth with stock! Though, as one person pointed out already, "stock" refers to the bone that is put in the water, the vegetarian broth and stock are also very different...stock is much richer.
made home made stock before, and there is definitely a difference, but since you seem to be busy, maybe this quick-fix option is a good bet for you! I would beware of any recipe that calls for multiple cups of water rather than stock or broth as the base. The stock/broth add a "simmered all day" flavor that you just can't get otherwise (as Rachel Ray likes to say, anyway).
Good luck! If you'd like some good soup cookbook recommendations, I'd be happy to help (I'm also a bit of a soup fanatic!).