I bring my vandas indoors whenever it gets below 50 degrees. I got that temperature from a book on vandas by Dr. Martin Motes. Not all vandas are the same though. Some are more sensitive to cold than others. I was told that blue/purple vandas can withstand cold weather better than yellow or red because of the species in their background. But the only vanda I have that has suffered significant cold damage was a blue vascostylis. It lost several leaves at a temperature where none of my other vandas suffered at all. I can't remember what the temperature was.