be on vacation是正在度假 go on vacation/take a vacation/go for vacation都是去度假 可以混用其中go for vacation 较随意可通过“go for a walk去随便走走”来理解 do for vacation为度假做准备 引申为去度假
What are you doing for vacation?是在为度假做什么准备
What are you doing on vacation?是在度假时做了什么