Place chicken thighs into a large saucepan, pour in water, and bring to a boil. Reduce heat to low and simmer until chicken is tender and no longer pink inside, 10 to 15 minutes. Drain, retaining chicken stock.
Preheat oven to 375 degrees F (190 degrees C).
Cook and stir green bell pepper and onion in 1 cup retained stock in a saucepan over medium heat until vegetables are tender, about 10 minutes. Cook vermicelli pasta with remaining 2 cups chicken stock in a separate pan over medium heat, stirring occasionally, until pasta is tender and stock has been absorbed, 12 to 15 minutes.
Combine cooked chicken, bell pepper, onion, cooked pasta, cream of mushroom soup, Worcestershire sauce, Cheddar cheese, salt, and black pepper in a large bowl; pour mixture into a 9x13-inch baking dish.
Bake in the preheated oven until casserole is bubbling and top is lightly browned, 20 to 25 minutes.
