In Dutch oven or large saucepan, bring 6 cups water to a boil. Meanwhile, cut bell peppers in half lengthwise; remove seeds. Place peppers in boiling water for 2 minutes. Drain; rinse with cold water until cool.
In large bowl, combine vinegar, chili powder, salt and oil; blend well. Add all remaining ingredients except lettuce and bell peppers; blend well.
Spoon bean mixture into pepper halves. Serve immediately on lettuce-lined plates, or refrigerate until serving time.