Cook pasta using package directions. Make sauce and veggies while pasta cooks.
In a medium bowl, combine peanut butter and warm water. Stir into a smooth, thin sauce.
Add soy sauce, vinegar, and sugar. Mix until sugar dissolves. If using, stir in red pepper flakes.
In a microwave-safe bowl, add thawed frozen veggies and ½ cup water. Steam in microwave for 3-5 minutes. Drain any excess water.
Pour peanut sauce and steamed veggies over cooked, drained pasta. Toss to combine. Serve warm or cold.
You can add chicken or tofu.