Foods that have a high acidity tend to trigger heartburn. Vegan chili soup contains natural ingredients that are acidic. These include onions, garlic, vinegar, chilies, and tomatoes.
If you have a sensitive stomach, you might want to go easy on the spice. Try adding some vegan yogurt or vegan sour cream to your vegan chili soup, or choose a variety with milder chilies.
There are some tricks for making chili less acidic, including adding ¼ of baking soda per serving, a dash of agave syrup, or some shredded carrot. If you are trying to limit your intake of acidic foods for health reasons, you may want to speak with a nutritionist or medical professional.