Cook the Rice Noodles: Prepare the rice noodles according to the package directions, keeping in mind that cooking times can vary depending on the type of noodle. Once cooked, drain thoroughly and set aside.
Blend the Spice Paste: In a small food processor or blender, combine the chopped chiles, lemongrass (if using), garlic, ginger, curry powder, and 1 tablespoon of coconut oil. Blend the mixture into a smooth paste. Add the cilantro leaves and lime zest, then pulse again to incorporate. Reserve the paste for later use.
Sauté the Eggplant: Heat 2 tablespoons of coconut oil in a large skillet (about 12 inches wide) over medium-high heat. Add the cubed eggplant and sauté until golden brown and tender, which should take approximately 10 minutes. Transfer the cooked eggplant to a plate and set aside.
Cook the Onions and Build the Sauce: In the same skillet, add the sliced onions and cook over medium heat until they soften and become translucent, about 7 minutes. Stir in the spice paste you prepared earlier, and cook for 1 minute to release its flavors. Pour in the stock and coconut milk, stirring to combine, and bring the mixture to a gentle simmer.
Simmer the Vegetables: Return the sautéed eggplant to the skillet. Add the halved cherry tomatoes and fish sauce, stirring to mix everything well. Allow the mixture to simmer gently for 7 to 10 minutes, or until the eggplant is very soft. If you're using fresh corn kernels or baby spinach, stir them in during the final minute of cooking. Adjust the seasoning with lime juice to taste, and sprinkle in Vietnamese coriander or basil, if using.
Assemble and Serve: Divide the cooked rice noodles evenly among serving bowls. Ladle the curry sauce with the vegetables over the noodles, making sure to distribute the ingredients evenly. Garnish each bowl with extra fresh cilantro or other herbs as desired, and serve immediately.