There are a few ways to thicken your chili. First, try simmering on low heat until it reaches your desired thickness. If that isn't working, try mashing some of the beans using a potato masher or fork. Alternatively, you can add tomato paste. Just stir in a few tablespoons, and adjust the spices (as tomato paste can be acidic). This will help thicken your one-pot vegan chili.