Cut the tops off the lemongrass stalks, about 6 inches from the base. Bruise the tops with something heavy, like a meat mallet, and reserve for the broth. Peel off 2 or 3 of the woody outer layers from the base stalks until you reach the smoother, more tender core. Roughly chop the stalks and put in a food processor along with the garlic and shallots. Pulse about 20 times until everything is finely chopped, but not a paste.
Heat the vegetable oil over medium heat in a large saucepan. Add the chopped lemongrass mixture and cook until soft and starting to brown, about 6 minutes. Add the crushed red pepper and stir for 1 minute. Add 2 tablespoons fish sauce, 4 teaspoons sugar and 1/2 teaspoon salt. Cook, stirring occasionally, until the mixture has thickened slightly, a spoon dragged across the pan leaves a trail and the oil is starting to separate from the solids. Remove all but 2 tablespoons of the lemongrass mixture from the pan and reserve.
Add the beef broth, 4 cups of water, and the bruised lemongrass tops to the saucepan. Bring to a boil and reduce to a simmer. Cook until it tastes like lemongrass, 15 to 20 minutes.
Meanwhile, bring a large pot of water to a boil. Turn off the heat and add the vermicelli noodles. Cook, stirring periodically, until tender, 6 to 8 minutes. Drain very well and divide among 4 large soup bowls.
Add the remaining tablespoon fish sauce, 1/2 teaspoon sugar and 3/4 teaspoon salt to the beef broth. Remove the lemongrass tops. Add the steak slices and gently simmer until no longer pink, about 2 minutes.
Top each bowl with slices of cooked beef, the white onion and cilantro. Pour hot broth into the bowls to completely cover the noodles.
Serve with the reserved lemongrass-chile mixture and suggested garnishes on the side.
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