Return the skillet to high. Pour in the stock, soy sauce, and fish sauce. Bring to a boil. Add the vermicelli and reduce heat to medium. Simmer, tossing constantly until noodles soften and liquid reduces to about 1/4 cup, about 5 minutes. Return the shrimp mixture to the pan. Cook, tossing, until most of the liquid has evaporated, 3 minutes. Add pepper. While noodles are still warm, toss in half the lemon-chili dressing. Let cool.