Remove “tenders” from chicken if necessary (see above note). Pound breast halves inch thick between 2 sheets of plastic wrap. On a work surface arrange 1 breast half, smooth side down, with a long side facing you and put 1 scallion (or 2 if very thin) lengthwise on chicken. Roll chicken around scallion and tie with kitchen string at 1-inch intervals. Trim scallion flush with chicken. Make 7 more rolls in same manner. Mince garlic and in a shallow baking dish just large enough to hold rolls in one layer stir together garlic, soy sauce, rice vinegar, and sesame oil. Add rolls, turning them to coat. Marinate negimaki, covered and chilled, at least 4 hours and up to 1 day. Coarsely chop bell pepper and in a blender puree with white vinegar. Transfer mixture to a small saucepan and stir in sugar, red pepper flakes, and salt to taste. Simmer sauce 5 minutes and cool. Sauce may be made 1 week ahead and chilled in an airtight container. Spread sesame seeds on a sheet of wax paper. Remove negimaki from marinade, letting excess drip off, and roll in sesame seeds to coat. In a large skillet heat 2 tablespoons vegetable oil over moderate heat until hot but not smoking and cook negimaki in 2 batches, turning them occasionally, until cooked through, about 10 minutes, adding remaining tablespoon vegetable oil to skillet as necessary. Transfer negimaki as cooked to a cutting board and cut crosswise into 1/2-inch-thick slices, discarding string. Push skewers through sesame seedcoated edge of slices. Serve negimaki warm or at room temperature with dipping sauce.