呵呵 ,for vacation是“为了度假”的意,例如,
I'll ask for three days leave for vacation.我将请三天假去度假.
on vacation 是在度假的意思,例如,
liming will be back in three days for he is on vacation.李明在度假,要三天后回来.
呵呵 说了这么多,不知道你是否明白了~~~~~~~~