Place ribeye (or any steak) in the freezer for 20-25 minutes. Remove from freezer and thinly slice against the grain.
In a bowl or shallow dish, add the ribeye, lemongrass, garlic, onion, oyster sauce, fish sauce, sugar, and ground black pepper. Mix until everything is well combined.
Marinate for 30 minutes.
Preheat a large cast iron skillet or wok over medium-high with 1 tbsp avocado oil.
Add the beef and onions to the skillet, spread out the beef so a majority of it is touching the skillet.
Let sit for 2-3 minutes or until it starts to char and then toss until cooked through.
Add all ingredients to a small saucepan.
Heat everything through until sugar has dissolved.
Remove from heat and set aside.
Prepare vermicelli noodles according to package.
Add vermicelli noodles to a bowl and top with Vietnamese lemongrass beef and the different toppings.
Dress the noodle salad with nuom mam cham.