In a small bowl, combine the vegetable and sesame oils. Heat a 10-inch cast-iron skillet over moderately low heat. Dip a paper towel in the oil and carefully wipe the inside of the skillet. Using a ladle, stir the batter and pour 1/4 cup into the center of the skillet. Working quickly, spread the batter out evenly to make an 8-inch pancake; move the back of the ladle in a spiral motion from the center. Fill in any large holes with a little extra batter. When the top looks dry and the underside has browned, about 1 minute, turn the dosa and cook until lightly browned, about 30 seconds. Transfer to a platter and continue making dosas, wiping the skillet with oil each time. Serve as soon as possible.