Shell whole shrimp, leaving tails on. Save shells and heads to make stock: Boil in 4 cups water for 15 minutes, strain, and add salt to taste. Bring shrimp stock, garlic, fish sauce, and Tom Yum paste to a boil. Add tofu and simmer for about 5 minutes. Add raw shrimp, mushrooms, coconut milk, and green onions, continuing to simmer for about 5 minutes, depending on the size of the shrimp - but don't overcook shrimp. Add vermicelli during the last 2 minutes. Serve immediately.*(If Tom Yum sauce is unavailable, make your own: Combine red chili paste to taste, 2 teaspoons chopped lemon grass, 2 teaspoons lime juice, 1 teaspoon salt, and 1 teaspoon sugar).