Meatballs: Place bread in large bowl. Moisten with 1/4 cup (50 mL) of the broth. Let stand 5 minutes. Using fork, stir to form paste. Add ground sirloin, coriander, ginger, onion, garlic, salt and sugar, using wet hands to gently work together. Shape into about thirty 1-inch (2.5 cm) balls; cover and refrigerate. Make ahead: cover and refrigerate for 1 hour or up to 1 day.
Soup: Meanwhile, bring remaining broth and chili flakes (if using) to boil in large saucepan or Dutch oven over medium-high heat. Add vermicelli. Reduce heat to simmer; cook for 5 minutes. Stir in lettuce and red pepper.
Slip meatballs into broth a few at a time. Without boiling, bring back to a simmer; cook, covered, until meatballs are thoroughly cooked, about 10 minutes, or until the temperature of several meatballs reads 160° F (71° C) when tested with a digital instant read thermometer. Stir in coriander and green onions. Use tongs to transfer meatballs, pasta and vegetables into eight soup bowls; ladle broth over each. Serve each with a lime wedge and splash of soy sauce.