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.
I got this recipe from the AllYou magazine several years ago, and have kept this recipe in my binder since then. It's yummy and easy. If you thought it was bland, add a little salt. It depends on the kind of salsa used because some have lots more salt than others. I've had it turn out bland before when I used different salsas.
I added chopped onions, cumin, brown sugar, liquid smoke, 3 cloves of garlic, garlic powder, salt, pepper, and beef bouillon to the recipe. I also reduced the salsa to 16 oz. I poured the sauce over the meat. I cooked it on low for 12 hours and then left it on warm for five hours. It was WONDERFUL!!! So tender and flavorful! Served with chipotle corn and fresh guacamole. YUM!!