Arrange peppers on a baking sheet coated with cooking spray. Broil 6 minutes or until blackened in spots, turning occasionally. Place peppers in a 6-quart electric slow cooker. Add vinegar, onion powder, garlic powder, and red pepper; stir to combine. Stir in 1/2 teaspoon salt, black pepper, and chopped pepperoncini.
Heat a large skillet over medium-high heat. Coat pan with cooking spray. Add beef; cook 5 minutes, turning to brown on all sides. Place beef in slow cooker. Cover and cook on LOW for 8 hours or until beef is very tender. Remove beef from cooker; shred with 2 forks. Stir beef and remaining 1/2 teaspoon salt into cooking liquid. Divide beef mixture evenly among sandwich rolls, spooning out with a slotted spoon. Serve any extra sauce for dipping.