IMO, you should feed your fish every day. It's really not natural, or healthy, for them to only eat every other, or every third day. I have two betas, and they are in bowls that look to be about the size of your vase. I don't have live plants in them, however. I feed my fish every day, sometimes twice per day. They always eat it all. I change their water once a week. Perhaps you could try feeding him every day and see how it goes?