1. In a 3- to 4-quart pan over medium-high heat, cook onion and garlic in olive oil, stirring often, until onion is limp and starting to brown, 6 to 8 minutes.
2. Add beans, tomatoes and their juice, cumin, and 1/2 cup water; bring to a boil, then reduce heat and simmer, stirring occasionally, to blend flavors, about 15 minutes.
3. Stir in cilantro, chipotle purée, and rice vinegar. Add salt to taste. Spoon chili equally into four bowls and top each with 1 tablespoon sour cream and, if desired, with tomato salsa to taste.
To make chipotle purée, whirl a 7-ounce can of chipotle chilies, including the adobo sauce, in a blender or food processor until smooth. Scrape into a plastic container and store airtight in the refrigerator up to 1 week or in the freezer for several months.