In a small bowl, zest and juice 2 of the limes. Whisk in 1 Tbsp each of fish sauce and sugar, and all of the oil and garlic.
Add steak to resealable bag and pour marinade over top. Seal and turn to coat steak. Let sit and marinade for at least 15 minutes.
Meanwhile, heat a large pot of water and cook rice noodles, according to package directions, until al dente. Drain.
While noodles are cooking, zest and juice remaining two limes and whisk in remaining fish sauce, sugar, 1 Tbsp water, and red pepper flakes. Pour over rice noodles and toss to coat.
Heat cast iron skillet over medium-high heat. Add steak to pan, pouring remaining marinade over top of steak, and cook, turning once, for about 10-15 minutes per side; or until desired doneness is reached. Remove from heat, cover, and let rest for 5 minutes.
Using the same pan that you cooked the steak in, over medium-high heat, add carrots and cook for 4-5 minutes, stirring occasionally until slightly soft.
Add sliced peppers and asparagus and cook additional 3-4 minutes, until crisp-tender. Season with salt and pepper and remove from heat.
Slice the steak across the grain and stir in with cooked veggies. Add noodles to bowls and place steak/veggie mixture on top.
Garnish with chopped peanuts, mint leaves, and extra limes. Serve immediately.