Sounds like it is a watering issue instead of potting issue. I don't believe cork will hold any more moisture than charcoal. You could try some redwood bark or perhaps coconut husk chunks, but the best bet in my opinion would be to increase watering, make sure you vanda gets lots of air movement, and see if it doesn't improve. Vandas like to drink and feed, but need to dry off quickly. They need good humidity as well.