In a medium bowl, combine 2 cups orange marmalade, 2 cups BBQ sauce, 1 cup sweet chili sauce, the zest of one orange, and 1 tablespoon white vinegar; stir until the mixture is evenly combined.
Spray the inside of the slow cooker insert with cooking oil or nonstick cooking spray.
Add the entire 32‑ounce bag of frozen, fully cooked meatballs (about 40–50) to the slow cooker.
Pour the sauce over the meatballs and gently stir once or twice to coat the meatballs evenly.
Cover the slow cooker and cook on HIGH for 2–3 hours, until the meatballs are heated through and the sauce is bubbly.
Stir gently, then garnish with diced green onion before serving.