Winters in Hong Kong are mild, but it sometimes go as low as 50's or lower. When that happens I bring the smaller ones indoors if I can find a space for them which is not easy considering the kind of housing we have here. The rest I just wrap them with clear plastic bag but only if its windy. Make sure they are dry though. Someone told me before that its not the cold temp that kills, its the cold dry wind. So even if its in the 50's I wouldn't worry much about them if its not windy. The vandas can withstand being outdoors also provided they were given time to acclimatize. If not, bag' em. Having said, don't take risk with plants that didn't had time to acclimatize to the cold. Bring them indoors.