Place pork in 3- to 4-quart slow cooker. Sprinkle with fajita seasoning. Top with salsa. Cover; 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. Increase heat setting to High. Cover; cook 30 minutes longer or until mixture is hot and vegetables are tender.
Using slotted spoon, place 1/2 cup meat mixture in each warm tortilla. Sprinkle with cheese. Serve with sour cream.