Scrub clams under running water. Place in a bowl, cover with water and add salt generously to the water. Set aside to purge clams for about 20 minutes. If clams are too dirty, repeat this process. After that, give them a quick rinse.
Slice pineapple into 1/3 inch thick slices. Cut tomatoes into wedges. Thinly slice scallion and roughly chop Vietnamese coriander.
Bring 2 1/2 cups of water to a boil in a pot over medium high heat. Add clams, cover the pot. When the shells are open, remove clams immediately from the pot. Skim off foam from the cooking liquid if there's any.
Remove the pot from the heat. Let all the impurities sink to the bottom of the pot. Carefully pour the cooking liquid into another clean pot without pouring the impurities. When you find it impossible to continue pouring without letting the impurities poured in, discard the remaining liquid.
Place the new pot with clear cooking liquid on the stove and bring to boil. Add pineapple and tomatoes, reduce heat to a simmer and cover and cook. Meanwhile, pick the clam meat from the shells, set aside.
When tomatoes are softened and you start to smell the pineapple scent, about 5 minutes or so, turn up the heat and when it starts to boil, add fish sauce and bean sprouts. Stir and when it starts boiling again, add clam meat and herbs.
Adjust seasoning to taste, then transfer to serving bowls and serve hot.