Drain, rinse and grind both the soaked grams separately, to a smooth paste. Mix them in a large bowl. Leave this paste to ferment for 8 hours. It will double in volume. Add ginger-green chilli paste and salt and mix well. Scoop batter into the wells of an idli pan and steam. They are ready when a toothpick inserted in the centre comes out clean. Cool them completely. Alternatively, you may make them in a paniyaram pan, by spooning some batter into each well, which has been greased using a peanut butter potli, covering and cooking for a minute or two. Turn the vadas over and cook on the other side for a minute or two, until they are evenly brown on both sides. If cooked in a paniyaram pan, soak the vadas in water for 2 minutes, drain and squeeze out the excess water.
Arrange the vadas on a serving platter, pour vegan curd on top and serve drizzled with date-tamarind chutney, garnished with coriander leaves and sprinkled with cumin powder, chilli powder and salt.