Heat oil in a small pot over medium-high heat. When it shimmers, add lemongrass, ginger, and chile and cook until fragrant, about 1 minute. Carefully add stock, coconut milk, and sugar and bring to a boil.
Reduce heat to low and add tofu, peas, and snow peas and cook until snow peas are cooked through, about 4 minutes.
Remove from heat and stir in basil, lime juice, and fish sauce. Serve.