Soak the rice and all the lentils/ grams in hot water for about 20 minutes.
Keep aside a tablespoon of the bengal gram, strain and blend the lentils and rice with red chillis, asafetida and salt into a very coarse thick batter, using minimum water.
Add the curry leaves, the finely chopped coriander leaves and the soaked bengal gram.
Heat the oil in a wok to fry the cakes. Take a square piece of plastic sheet, or a betel leaf or, a banana leaf moisten with water and holding it in the palm, pat the batter on it. Or, moisten the palm of your hand and prepare the patti.
Make patties of two-inch diameter and drop gently into the hot oil. Lower the flame to medium and maintain the same temperature throughout. You can make three to four vadais at a time. With a slotted metal ladle, turn the vadais gently until golden in color. Drop them on a kitchen paper to drain the excess oil.