1-2 hours before cooking, pat dry and put fish fillets in the freezer to make them cold and firm.
Use a damp piece of paper towel to gently clean kombu. Please do not wipe off the white powder on the surface of kombu, it contains a lot of umami. Put kombu and water in a pot and place over medium heat. Bring it to a bare simmer then discard kombu.
Add katsuobushi to the stock, bring it to a boil and then turn off the heat. Let stand for 10 minutes, then remove katsuobushi from the broth.
Turn on the heat. Quarter tomatoes. Add tomatoes and 1 1/2 tablespoons of fish sauce to the broth. When it starts to boil, lower the heat, cover the pot and let it simmer.
While kombu is being simmered in the step 2 above, remove fish fillets from the freezer, cut into smaller chunks. Place fish and all other ingredients to make fish cake in a food processor and process into a sticky mixture.
Transfer fish paste from food processor to a mixing bowl. Knead for about 2 minutes into a smooth, dense and springy fish paste mixture.
While steeping katsuobushi in the stock in the step 3 above, take 1/2 tablespoon to 2 teaspoons of fish paste to form thin patties.
Place a frying pan over medium high heat. Add oil and fry fish cakes until the edge is crispy and both sides are golden brown (about 2 minutes each side). Transfer to a clean plate lined with paper towel.
Prepare vermicelli noodles according to package instructions.
Taste broth and adjust seasoning as needed. Place vermicelli noodles in serving bowl, top with fried fish cakes and scallion, cilantro and dill. Ladle the hot broth over noodles and enjoy.