Place red onion slices into a bowl and cover with ice water; stir in vinegar.
Place lentils into a saucepan with water and bring to a boil; reduce temperature to low and simmer until lentils are tender and still keep their shape, 5 to 10 minutes. Stir 1 teaspoon salt into lentils and let sit off the heat for 1 minute; drain and transfer lentils into a salad bowl.
Drain onion slices; gently fold onion, cherry tomatoes, parsley, cilantro, lemon juice, and olive oil into lentils. Season with salt and black pepper.
Heat a heavy skillet over high heat and pour in vegetable oil; remove skillet from heat and stir in cumin seeds, garlic, and chile pepper. Cook and stir until fragrant, about 30 seconds; stir in tamarind paste and turmeric. Mix well for 30 seconds. Pour tamarind mixture over lentils, toss to coat, and serve.
Be sure not to overcook the lentils; they should be just tender and not broken apart.