Bring a large pot of salted water to a boil. Add the peas and cook until bright green and crisp-tender, about 5 minutes. Drain and run under cold water until the peas cool to room temperature; pat dry. Heat the olive oil in a large skillet over medium heat. Add the pancetta and cook, stirring, until golden, about 5 minutes. Add the leek, garlic and red pepper flakes and cook, stirring, until the leek is soft, about 3 more minutes. Transfer to a large bowl. Add the peas and 1/2 teaspoon salt and toss. Add the vinegar, season with salt and pepper, and toss again. Serve at room temperature. Tip: Snap peas have tough strings on both sides of the pod. To remove them, pinch off the stem and pull the string; repeat on the other side.