Layer the sliced peppers and onions in the bottom of a 5-quart slow cooker.
Place the thinly sliced beef on top of the vegetables.
In a small bowl, whisk together the water, red wine vinegar, lime juice, cumin, chili powder, salt, garlic powder, black pepper, and cayenne pepper. Pour this mixture evenly over the beef.
Cover the slow cooker and set it to low. Cook for 8 to 10 hours, until the beef is tender and infused with the savory seasoning.
Once done, use a slotted spoon to scoop 3/4 cup of the beef and veggie mixture onto each tortilla.
Top with salsa, cheddar cheese, and fresh cilantro. Roll up your fajitas, serve, and enjoy!