Any leftover dal vadas can be refrigerated for upto 5 days. Just make sure you use an airtight container or a Ziploc bag to store them and reheat them in the microwave to enjoy the perfect texture and flavor.  You can also make the batter in a big batch (minus the onions and other veggies), and refrigerate it. Then, when you want to fry the vadas, you have the batter ready to be used.