Given that it's shedding bottom leaves it's probably nothing to be too concerned about. From what I can see in the pics the plant looks reasonably healthy. My vandas tend to shed more leaves this time of year, and I believe it's due mostly to the cooler and dryer conditions of winter. Some questions:
-What are the temps you are growing it in?
-Temp of the water you're using?
-How much light are you giving it?
Sounds like you're doing everything about right. I fertilize more often, once a week this time of year and twice a week in the summer. Vandas are heavy feeders and generally do better with more fertilizer than most types of orchids. I also have found them to be very forgiving as long as they're kept warm, and given plenty of light and water.