Heat the oil in a large skillet over medium heat. Add the onion, garlic, chili powder, and cumin and sauté until onions are soft.
Add the tomatoes and 1 cup water. Bring to a simmer, partially cover with a lid, and cook 10 minutes.
Add the beans and return to a simmer. Continue cooking, uncovered, for 15 minutes or until the chili is thick.
Remove from heat and stir in the vinegar. Season to taste with salt and pepper.
Meanwhile, prepare the polenta according to the package directions; stir in the butter. Season to taste with salt and pepper.
Divide the polenta between 4 bowls. Ladle the chili over the top. Spoon a little salsa on top, if desired.