Set Instant Pot to saute mode, add the oil. When hot, add the shallots and garlic and stir fry till fragrant. Add the sliced tomatoes and sauté for 2 minutes. Remove the sliced tomatoes and set aside (the shallots and garlic remain in the instant pot). Cancel the saute setting.
Add the chicken broth, sugar, ketchup, Bun Rieu seasoning cube, anetto oil (if used), fish sauce. Close the Instant Pot lid, the knob is in shut position, and set to "Soup / Broth" function (cooking time is usually 20 minutes).
In the meantime, wash the deveined prawns (I rinse them with a little white vinegar and water), pat dry. In a food processor (I use a Magic Bullet), add the prawns and shallots and pulse 4 to 5 times till the prawns form a paste. Remove to a bowl with a spatula, add the minced pork, the crab meat, and half can of crab in spices, seasoning, stir well. Add 2 eggs, mix well. Cover and refrigerate till ready to use.
Prepare the vermicelli according to the package instructions or use my time-saving tip if you wish. Divide the vermicelli into 4 to 5 bowls. Divide the fried tomatoes wedges among the bowls.
Wash and display the herbs. Slice the limes and Thai chilis
If using the water spinach stems, you need to split the stems - There's a Vietnamese knife gadget for this purpose, or use a peeler. Soak the split stems in water with a squeeze of lemon to keep them green and fresh, until ready to serve.
When the soup is ready, remove the lid, taste and adjust seasoning. The broth should taste a little salty, sweet and sour. Set on saute function till the broth is boiling, add the fried tofu, and with a large spoon, scoop a ball of the crab, shrimp and meat mixture and add to the boiling broth, repeat for the remaining mixture (there should be around 12 mixture balls). At first the mixture balls will sink in the boiling broth. When cooked, they will rise to the surface, it takes usually 15 to 20 minutes. Test 1 ball for doneness.
Laddle the soup, tofu and crab and meat balls into each bowl of vermicell. Serve piping hot with the garnish.