Combine first 3 ingredients in a Dutch oven; bring to a boil. Reduce heat; simmer, partially covered, 30 minutes or until tender. Remove from heat.
Heat butter in a large skillet over medium-high heat. Add onion; sauté 5 minutes. Add Swiss chard and pepper. Sauté 3 minutes or until wilted; stir in vinegar. Add onion mixture to peas; stir.