Hi Beezer34,
Most vandas should have leaves along the entire length of stem, although a few species do not. A couple of reasons that they defoliate are a lack of water, and temperatures lower than 50 degrees F.
Under ideal conditions, vandas should be watered daily: a good watering until the water is running off of the roots, and then a repeat watering about 10 minutes later. On really hot days, a second watering late in the morning or early afternoon will not hurt, so long as there is enough daylight time left in the day for the roots to dry.
I hope this helps.