In a medium bowl whisk together soy sauce, brown sugar, rice wine vinegar, garlic, ginger and red pepper flakes.
Heat 2 tablespoons vegetable oil in a large skillet over medium-high heat. Add half of the steak strips and cook for 3 minutes. Remove from the skillet and set aside.
Add remaining 1 tablespoon of vegetable oil to the same skillet. Add remaining steak strips and cook for an additional 3 minutes. Remove from the skillet and set aside with the other steak strips.
Add all of the steak strips back to the skillet along with green onions. Pour in soy sauce mixture and bring to a simmer.
Transfer everything to a slow cooker and cook on low for 5-6 hours or on high for 2-3 hours.