Bring a large pot of salted water to a boil for the rice noodles. Add the rice noodles to the boiling water, stir, and remove from heat. Cook, stirring occasionally, until the noodles are tender, about 10 to 12 minutes. Drain and rinse under cold water to stop the cooking process. Toss with 1 tsp vegetable oil to prevent sticking.
Peel and dice the onion. Peel and mince the garlic and ginger. Cut the lemongrass in half and carefully crush the stalk with the flat side of a knife. Peel and thinly slice the carrot into rounds. Thinly slice the scallion. Drain the tofu, pat dry with paper towels, then cut into small cubes. Halve the lime and cut half into wedges.
Place a medium saucepan over medium-high heat with 2 tsp vegetable oil. Once hot, add the diced onion and cook until softened, about 3 to 5 minutes. Add the minced garlic and ginger and cook until aromatic, about 1 minute. Add the sliced carrot, crushed lemongrass, tamari, coconut milk, 2½ cups water, and ½ tsp salt. Bring to a boil, reduce heat to low, and simmer until Step 6.
Place a large nonstick skillet over high heat with 1 tbsp vegetable oil. Once hot, add the cubed tofu and sprinkle with salt and pepper. Cook tofu, tossing occasionally, until browned in places, about 3 to 5 minutes.
Once the tofu is crispy, add as much Sriracha as you’d like to the skillet and reduce heat to medium. Toss to coat evenly, about 1 to 2 minutes.
Remove and discard the lemongrass from the coconut broth. Remove broth from heat, add the juice from half the lime. Divide the rice noodles between large bowls and ladle the coconut broth over the noodles. Top with Sriracha tofu, sliced scallion, and serve with lime wedges.