Can you post a photo? Is there any bug on the plants. Sometimes scale will attach vandas very near the vertical stem where the leaf attaches. Also, when did the leaves turn yellow? Immediately after the cold or just recently? If it has been awhile, it was probably not the cold. How do the roots look? Are they thick? Thin? Brittle or Fleshy?
Some additional info or a photograph would help us make some recommendations.