Spoon 1 cup of the salsa into a small bowl and set aside.
Add remaining salsa, chili powder and vinegar to a slow cooker, stir to combine ingredients. Add beef, cover slow cooker, and cook on low for 10-12 hours. Shred meat using two forks then place in a serving bowl.
Place lettuce and sour cream in separate serving bowls.
Remove pit from avocados, peel and dice. Place diced avocado in a serving bowl.
Place serving bowls and warmed tortillas on table, create tacos and enjoy.