Place pork in 3- to 4-quart slow cooker. Place onion on top. Mix barbecue sauce, salsa, chili powder and cumin in small bowl; pour over pork and onion.
Cover and cook on Low heat setting 8 to 10 hours.
Remove pork from cooker; place on cutting board. Shred pork, using 2 forks. Return pork to cooker and mix well. Stir in stir-fry vegetables and salt. Increase heat setting to High. Cover and cook 30 minutes or until mixture is hot and vegetables are tender.
Using slotted spoon to remove pork mixture from cooker, fill each tortilla with 1/2 cup pork mixture. Fold one end of tortilla up about 1 inch over filling; fold right and left sides over folded end, overlapping. Fold remaining end down. Serve with cheese, guacamole and sour cream.