Hi Yamuna,
I have tried freezing vadas for Dhahi bhalle. The reheating process is simple. You fill a skillet, with water. Arrange the frozen vadais, in a single layer, and heat it. Just before the water begins to boil, switch off the stove, flip the vadais, and close the skillet with the lid. After five min remove the vadais, squeeze well, add to yogurt mixture. They turn out pretty good.
To make medhu vadais crispy add a 1tsp of rice flour to 1 cup of urad dhal batter. I have refrigerated the vadais, and reheated them in the oven. Results were pretty decent. If you want the real authentic taste, fry it 1-2 hours before your company arrives. Hope this helps.
Suja Rajkumar