Spoon 1 cup salsa into a small bowl; reserve. In a slow cooker, combine remaining salsa with vinegar and chili powder.
Add beef, cover and turn heat to low. Cook until beef shreds easily with a fork, 10 to 12 hours. Shred meat, using two forks, and spoon into a serving bowl.
Preheat oven to 300F. Stack tortillas, wrap in foil and bake until warm, about 10 minutes.
Place sour cream and lettuce in bowls. Just before serving, pit, peel and dice avocado and place in a small bowl. Put out bowls (including salsa) and assemble tacos at table.
