Begin by preparing all vegetables and aromatics. Thinly slice or dice the white onion, grate the galangal or ginger, and mince the garlic and Thai chilies. If using fresh lemongrass, finely chop the stalk; otherwise, measure out the lemongrass paste.

In a large non-stick pot over medium-high heat, add the white onion, galangal or ginger, Thai chilies, lemongrass, and garlic. Stir frequently to prevent sticking, adding a small amount of water as needed. Sauté for 1-2 minutes until fragrant. Incorporate the Thai red curry paste and continue stirring for an additional 1-2 minutes until evenly combined.

Add the chopped shiitake mushrooms and sliced red bell pepper to the pot. Sauté for 5 minutes, stirring frequently. Add water in small amounts as necessary to prevent burning.

Pour in the vegan chicken broth and coconut milk. Reduce the heat to medium and bring the soup to a gentle simmer. Add the cubed tofu and cook for 6-8 minutes until heated through.

If including rice vermicelli noodles, cook them in a separate pot according to package instructions. Set aside.

Add the coconut sugar and lime juice to the soup, adjusting quantities to taste. Allow the soup to warm for an additional 1-2 minutes, then remove it from the heat.

Ladle the soup into serving bowls. If using noodles, divide them evenly among the bowls prior to adding the soup. Garnish with sliced green onions, fresh cilantro leaves, and additional lime wedges if desired.