In a bowl, add vinegar, soy sauce, fish sauce, maple, 2 tablespoons olive oil, and garlic (if using). Add the veggies and toss to coat. Add the herbs and gently toss to combine.
In a large skillet, over medium heat, warm about 1 tablespoon oil. Arrange the dumplings in a single layer and cook until golden brown on the bottom, about 2 minutes. Add ¼ cup water, then immediately cover with a lid, and reduce the heat to low. Simmer/steam for 6-10 more minutes or until warmed through.
Remove the lid, turn the heat back up to medium or medium-high and cook until the water has evaporated and the bottoms are crisp and golden, about 1-2 more minutes. Transfer to a cutting board and cut them in half or serve them whole.
Serve the dumplings over the slaw topped with avocado, sesame seeds, and chili oil (if using). Spoon the dressing from the bottom of the bowl onto the dumplings.