Heat the oil in a pot over high heat. Sauté sliced onion until fragrant. Add tomato wedges and sauté until they're about half cooked.
Add about 1.5 liters of hot water to the pot along with the tamarind pulp. Turn the heat down to medium and allow to simmer covered for about 15 minutes.
While the tomato broth is simmering, cook the vermicelli rice noodles according to the instructions. Rinse with cold water and strain.
Beat the eggs with the crab paste. Bring the broth to a rolling boil and gently add the crab paste egg mixture one ladle at a time. Allow the broth to come back to a gentle boil. The crab paste mixture will rise to the top as it cooks. Add the fried tofu and let it cook for about another five minutes.
Serve the broth over a bowl of the cooked vermicelli rice noodles. Garnish with chopped Vietnamese perilla and bird's eye chilis if you like it spicy.
Notes
*For the vermicelli rice noodles, I prefer the Vietnamese Bamboo Tree brand because the noodles aren't so thin that they fall apart in the soup. The broth doesn't require any seasoning because the crab paste is already adequately salty. But, if you'd like, you can add a little bit of shrimp paste when serving.I usually buy freshly fried tofu cubes and cut them into slices before adding to the broth.