Starting with a well made medu vada that is crispy on the outside and cooked well from the inside, squeezing the excess oil if any during the first soak in the diluted butter milk (when the excess oil is out of the way, we are left with a well cooked dough that got a well aerated weave like the honey comb) and later allowing enough time for the vadas to rest in the spiced yogurt will give you soft dahi vadas (similar to sponge that can absorb more liquid with time).