Slice flank steak against the grain on a 45-degree angle into ¼-inch thin strips. Transfer beef strips to a large bowl. Tip: to thinly cut the beef easily, freeze the flank steak for 20-30 minutes fully exposed to the cold air and cut with a sharp knife.
Add beef marinade ingredients into the same bowl with the beef strips. Mix with tongs or clean hands and then pack the beef into a ball so that the marinade can absorb into the meat. Set aside to marinate for 15 minutes.
Into a high power blender or food processor, combine satay sauce ingredients as listed and blend on high until very smooth. If you don't own a blender, finely mince the lemongrass, red chili, garlic and shallots and combine with other satay sauce ingredients in a bowl.
Heat vegetable oil in a large pan on medium-heat and fry marinated beef. Fry until the edges are brown and the meat is 75% cooked, about 5 minutes.
Then add in satay sauce and toss beef in it. Cook for 1-2 minutes until beef is cooked through and no longer red. Remove and set aside.
In a large pot filled with enough water, bring to a boil on medium-high heat. Boil rice vermicelli noodles until al-dente, about 2-3 minutes. Strain immediately. Rinse cooked noodles with cold running water and strain again. Divide your noodles into 5 bowls.
Divide cooked satay beef and place on top of cooked noodles.
In a large pot, combine chicken broth, oyster sauce, regular soy sauce and sesame oil. Bring to a boil on medium-high heat. Once it comes to a boil, divide and pour the soup over the noodles and enjoy hot!