When do you mention already booked travel plans to a prospective employer?
Wait until after the job offer is made, unless the amount of time you will be gone is much larger than a "normal" vacation length. If the employer wants you as an employee, having you gone for a time shouldn't break the deal. If need be, you can work this previously planned trip into negotiating overall vacation time and salary. I was offered a job one year in April but because of a required move, did not start until July. If you are the one they want for the job, it costs them more to find a better candidate than to let you take a trip early in your employment. Don't bring it up until you know they want you.