Medu Vada is a traditional favorite of south india that features as an important part of menu on festivals and puja days .The trick in getting perfect vadas is to grind the batter with very less water and then deep fry on medium flame . Its crispier from outside but very soft inside. Thats sign of a perfect vada. Some people do soak the Urad dal overnight. I would soak for an hour and grind it. Its completely optional to press a hole in the centre of vadas . Its just the way it is prepared by some professional cooks. I am not professional but I can say i got vadas in good shape after practising 2-3 times. So you also make it like small balls.