"on vacation" is correct.
� Common mistakes with in/on/at
o Not used with vacations, use �over� or �during� for specific vacations, but simply �on� for vacation alone.
� (Wrong) In summer vacation, I had a part time job.
� (Right) During summer vacation, I had a part time job.
� (Right) Over summer vacation, I had a part time job.
� (Wrong) I had a good time in vacation.
� (Right) I had a good time on vacation
Cheers --- Jayho