Bring a large pot of water to a rolling boil. Add the udon noodles and cook according to package instructions, usually 3-4 minutes for fresh or 8-10 for dried. Drain and set aside.
Heat your wok or deep skillet over medium-high heat until shimmering. Add a tablespoon of neutral oil, swirling to coat the surface.
Add the sliced bell peppers, julienned carrots, and zucchini to the hot pan. Stir-fry for about 2-3 minutes until vegetables are vibrant and just tender, with some crunch remaining.
Push the vegetables to one side of the wok. Add the minced garlic and grated ginger to the cleared space. Sauté for 30 seconds until fragrant, watching for a fragrant aroma and slight sizzle.
Pour in the soy sauce and rice vinegar (if using). Drizzle with the toasted sesame oil. Toss everything together gently, letting the sauce coat the vegetables evenly and turn glossy.
Add the drained udon noodles to the wok. Toss gently with tongs or a spatula for 1-2 minutes, allowing the noodles to soak up the flavors and heat through.
Stir in the sliced green onions and give everything one last gentle toss. Taste and adjust seasoning with more soy if needed.
Remove from heat and let rest for a minute to allow flavors to settle. Serve immediately, garnished with extra green onions or sesame seeds if desired.
