Combine the BBQ sauce, vinegar, sugar, Worcestershire sauce, and pepper in a bowl. Mix well.
Place the chicken in a slow cooker and then pour-in the sauce mixture.
Slow cook in low for about 5 to 6 hours. Shred the chicken on the 3rd hour and then continue to slow cook. Keep an eye on the cooker to ensure that there is still enough liquid.
Arrange the slow cooked chicken BBQ in the middle of each sub.
Serve. Share and enjoy!