Whisk together the fish sauce, soy sauce and vinegar, and set aside. Roughly chop the garlic and 3 of the chilies together. Smash the other two chilies with the flat of a knife, and set aside.

Put a wok (or a large frying pan) over medium-high heat; when its hot, add the oil, the garlic-and-chile mixture and the onion. Cook, stirring constantly, until the garlic is fragrant, about 30 seconds. Add the pork and a splash of the sauce. Cook, stirring to break up the meat, until the pork is cooked through, about 5 minutes.

Add the peppers and noodles. Turn the heat to high, and add almost all of the sauce (save a spoonful or two to add later if needed). Cook, tossing everything together and separating the noodles if necessary, until the noodles are coated in sauce and take on a slightly charred flavor from the wok. Taste, and add more sauce if needed. Toss in the basil and the smashed chiles, and serve immediately.