Preheat broiler. Place peppers on a baking sheet and broil for 4-6 minutes, turning as needed to char on all sides; remove the stems and transfer peppers to the bottom of the crock pot.
To the crock pot, add the vinegar, onion powder, garlic powder, red pepper,1/2 teaspoon salt, black pepper, and chopped pepperoncini; stir well.
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 and sprinkle with remaining ½ teaspoon salt. Cover and cook on low for 8-10 hours.
Remove beef to a cutting board and shred with 2 forks. Return beef to crock pot; gently stir to moisten meat with liquid before dividing evenly between the sandwich rolls and serving.