Place the tri-tip in a slow cooker.
In a medium bowl, whisk together the vinegar, soy sauce, minced garlic, four tbsp of lime juice, olive oil, salt, black pepper, white pepper, garlic powder, chili powder, oregano, cumin, and paprika. Whisk until well blended, then pour over the tri-tip. Turn over once to coat both sides.
Set slow cooker to low, cover, and cook for 8 hours.
In a small bowl, stir together one chopped white onion, cilantro, and two tbsp of lime juice. Cover and store in the refrigerator to use on tacos later.
Once the tri-tip is fully cooked, remove from slow cooker and cut into small pieces. (Insider tip: you can achieve an extra sear on the tri-tip by heating vegetable oil in a large skillet over medium-high heat. Pan sear while continually stirring until most of the liquid has evaporated.)
Warm the tortillas in a skillet for about a minute on each side to make them pliable. Arrange two or three tortillas on a plate, and lay a generous amount of taco meat over them and sprinkle of the onion relish. Add as much cheese as you like. Garnish with lime wedges and serve.
Don't have 8 hours? Skip the slow cooker and throw it on the grill.
Photo compliments of Christine Siracusa