Heat the oil in a large non-stick skillet over medium heat. Once hot, add the minced lemongrass, garlic, and chili. Sauté for about 2 minutes.
Finely chop the dry black mushroom and add to the skillet. Sauté for another minute.
Add the sugar, salt, soybean sauce, water, cooked chopped vermicelli, and diced tofu. Let simmer for about 10 minutes, stirring regularly to coat the tofu with the sauce. Let it simmer until the sauce has slightly thickened.
Top with chopped green onions and serve hot with steamed rice and/or your favorite greens! Tofu will keep for up to 3 days in the refrigerator.
You can use store-bought soybean sauce, however you will have to make a few adjustments since it's usually more salty and less sweet than the homemade version. Use only 1/2 cup of soybean sauce and increase the sugar (or maple syrup) to 2 tbsp.