I would go. You don't lose anything by going. Chances are that they are NOT going to be still looking when you come back, and if they are not that much in a hurry they may be able to wait for you.

They may ask you when can you start working, so tell them the date. Make sure the reason is something that sounds serious, not just vacations that you are unwilling to cancel just because they offer you a job. Express excitement at a prospect of working for them, and willingness to do everything you can from abroad, over the phone, fax internet, whatever, and you wish you could cancel or postpone, but that just can't be done. Perhaps it is a family matter, or something legal, there really are priorities that are higher than job, and you may not want to work for employer who thinks their deadline trumps your mom's surgery or whatever.

If it is a dream vacation don't tell them that. Don't have to tell them WHAT it is, don't have to LIE, make it vague, but important sounding.