Mix the zucchini and potatoes together, add salt and keep aside. Drain and set aside the water that will be released. You can use it while preparing the batter.
Add all the ingredients except the oil and make into a batter. (If water is needed add the zucchini water to the batter.) Heat oil in a pan, make thin vadais with the batter and fry them until they turn brown. (Do not make thick vadais as it will not get cooked on the inside.)
Fry them in batches, making sure not to overcrowd the pan.