elfshadow -
A good vegetarian soup stock will contain onions, garlic, carrots, and celery, but other things that really help it out can be any of the following - mushroom stems from white, crimini or portobellos (freeze them when you use the tops), the bottom bits of asparagus stalks (freeze those as well), tomatoes depending on the type of soup you are planning to make, and fresh or dried herbs of all sorts - parsley, basil, oregano, thyme, sage, black pepper, and sea salt or kosher salt. Things like cabbage or broccoli tend to sour a stock quickly if not in the soup itself, so they are not recommended for a stock. The same thing goes for leafy greens like spinach or chard. Chiles (unless dried) and bell peppers are also not good until you make the soup itself.
When I make a veggie soup stock, I also like to add a healthy dollop of good soy sauce for flavor, it really spikes up the taste.
But, remember the key thing about a stock - you can always add something more flavorful to it, but you cannot take away something that overpowers it - go lightly with seasonings on your stock itself, you can always jazz it up when you make the soup.