These Vietnamese appetizers are traditionally served with fresh herbs and rolled up in lettuce leaves. Serve the herbs and lettuce on a separate platter, and allow guests to roll their own.
Place vermicelli in a large bowl. Cover with hot water; soak 30 minutes. Place dried mushrooms in a small bowl. Cover with boiling water; soak 30 minutes. Drain vermicelli, and cut into 1/2-inch lengths; place in a small bowl. Drain mushrooms, and roughly chop; add to vermicelli along with onions, carrots, and scallions.
Whisk together eggs, fish sauce, garlic, sugar, salt, and pepper. Using a fork, mix chicken and pork into egg mixture. Add noodle mixture; mix until just combined.
In a small saucepan over medium heat, whisk cornstarch and 5 tablespoons water until mixture comes to a boil and looks like a smooth porridge, 1 to 2 minutes; remove cornstarch paste from heat.
Line a baking sheet with parchment. Place wrappers in neat stack; cut in half into 4-by-8-inch rectangles. Loosely cover with plastic wrap. Place 1 wrapper on a clean surface with narrow edge facing you. Place 1 tablespoon filling 1/2 inch in from narrow edge. Fold about 1/2 inch of right and left sides over filling, and roll wrapper up to form a cylinder. Use your finger to smear cornstarch paste along end of wrapper; gently press to seal. Transfer to prepared baking sheet. Repeat, using all wrappers and filling.
Heat oil (there should be enough to cover spring rolls) in a wok or large saucepan over medium-high heat to 375 degrees. measured on a candy or frying thermometer. (If using a wok, turn the burner grate over; place wok directly on grate so it is as close to flame as possible.) Carefully slip as many spring rolls as will fit comfortably into oil. Fry, turning often, until golden and crisp, about 3 minutes. Using a slotted spoon, transfer spring rolls to paper towels to drain. Repeat until all the rolls are cooked. Serve immediately. To eat, sprinkle inside of a lettuce leaf with herb leaves, wrap around a spring roll, and dip in dipping sauce.