Arrange a rack 4 inches from broiler and heat. Put shallots and ginger on an aluminum foil-lined baking sheet and broil, turning often, until blackened (15–20 minutes) and let cool.
Scrape peels off shallots and ginger. Halve ginger lengthwise, press each piece with the side of a knife to flatten, and set aside with shallots.
Heat fennel seeds, star anise, cinnamon, and cardamom in a small skillet over medium heat and toast, swirling pan, until fragrant (3 minutes). Transfer spices to a small bowl and set aside.
Place bones in a 12 qt pot and cover with cold water by 1 inch. Bring to a boil and cook for 3 minutes. Drain and rinse bones. Clean pot and return bones to pot along with reserved shallots and ginger, beef and cold water.
Bring to a boil. Reduce heat to medium-low and add reserved toasted spices, scallops and 2 tbsp salt. Cook, skimming surface, until beef is tender (1 1/2 hours).
Using tongs, transfer beef to a bowl of ice water and cool for 10 minutes. Drain beef and thinly slice crosswise. Transfer to a plate, cover and refrigerate.
Continue cooking broth for 1 1/2 hours more. Remove from heat and pour through a cheesecloth-lined fine strainer set over a clean 6 qt pot. Discard solids and skim fat from surface. Stir in fish sauce and scallion whites and keep hot.
Combine vinegar and chiles in a small bowl and set aside.
Pour boiling water over noodles in a medium bowl and let soak until al dente (10 minutes). Rinse noodles in cold water, drain and divide between 8 serving bowls.
Top each with chilled, cooked beef and raw sirloin. Top beef with onions, then scallion greens and cilantro. Season with pepper and then ladle broth over each serving, placing one white scallion piece in each bowl.