Cook ziti as directed on package. Drain.
Meanwhile, in 12-inch skillet, heat oil over medium-high heat until hot. Add onion; cook and stir 2 minutes. Add bell peppers; cook 3 to 5 minutes or until onion and peppers are crisp-tender, stirring frequently.
Add cooked ziti; stir gently to mix. Remove from heat; stir in pesto and vinegar. If desired, add salt and pepper to taste. Sprinkle with cheese.