First make the sauce. In a medium bowl, pour the boiling water over the brown sugar and stir to dissolve. Stir in the lime juice, vinegar, fish sauce, and sriracha. Add the chicken, stir to coat it, and place in the refrigerator to marinate as you prep the other ingredients.
Cook the noodles according to the package directions and set them aside.
In a wok or large skillet over high heat, warm the oil. Add the onion and cook, stirring frequently, for 1 minute. Stir in the garlic, then add the eggs and cook for 30 seconds, continuing to stir. Add the chicken and sauce, stirring and cooking until the chicken is no longer pink, about 8 minutes. Add the noodles and cook until warmed through, about 3 minutes.
Remove the pan from the heat and toss immediately with the vegetables. Serve with your desired garnish (or provide garnishes in small bowls at the table).