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 300°F. 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.
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