I recently found an interesting version of Dahi Vada on Manjula's Kitchen, where she mixed mung dal with udad dal to make the vadas. I immediately tried these @ home and was very happy with the results - melt-in-your-mouth dahi vadas. I know the above picture doesn't do justice to the ultimate dish, but don't let that put you off.
Here's how I made them:
For the vadas (about 20 - 25 pearl onion sized ones) -
1/3 cup udad dal
1 tbsp mung dal
Salt, pepper to taste
Soak the dals separately in water overnight. Next morning, mix and grind them together with 2 tsp of water (use water in which they were soaked). Take the batter in a mixing bowl. Whip it in a circular motion with a spoon for about 5 mins. The batter turns noticeable light and fluffy. If you drop a bit of it in a bowl of water, it will immediately float up. This means its really to fry. Add salt and pepper. Chopped green chillies and grated ginger can also be added if desired. I usually do not, in order to keep them kid friendly. Drop spponfuls of batter in medium hot oil and fry till light brown. Drain on paper towels. Next you can soak these vadas in warm salted water if desired, but I usually skip this step.
To make the dahi vadas, whip about 1.5 cups dahi with some water till smooth. Add salt, sugar. Pour over the vadas. Garnish with red chilli pwd and cumin pwd to taste. Top with chopped cilantro. You can also add some green chilli chutney and tamarind chutney to spice up these vadas.