Vegetable stock is a delicate substitute for chicken stock in soups.  It can actually be very inexpensive if you think ahead: Instead of using whole vegetables, save your cleaned trimmings and scraps (potato skins, onion skins, carrot tops and peels, celery leaves, etc.) in the refrigerator or freezer.  Use about 2 cups of vegetable scraps for every quart of water.  Of course, you can also use fresh vegetables as below.  Do not overdo strongly flavored vegetables such as members of the cabbage family.  Avoid adding bell peppers, which impart an off flavor.  For extra richness, save the water when cooking vegetables to use as part or all of the stock’s cooking liquid.