Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add bell pepper, and cook, shaking skillet often, until pepper blisters, 2 to 4 minutes. Stir in 1 tablespoon soy sauce. Transfer to a plate; wipe skillet clean.
Place each beef slice between plastic wrap; pound lightly until 1/8 inch thick. Season with salt and pepper. Divide bell pepper and scallions among slices. Roll tightly into bundles; secure each with a toothpick.
Heat remaining tablespoon oil in same skillet over high heat. Working in two batches, sear rolls until browned on all sides, 3 to 4 minutes. Transfer to a plate; cover to keep warm. Wipe skillet clean.
Add 2 tablespoons water, remaining 2 tablespoons soy sauce, vinegar, and sugar to skillet. Cook over medium heat, stirring, until sauce is reduced and syrupy, 2 to 3 minutes. Remove toothpicks. Drizzle rolls with sauce.