Already Planned my vacation

Sooo... About a year ago I planned a vacation with my family to go back to my mother's home country that she hasn't been back to in 20 years. We are supposed to go at the end of March. Now the day after Christmas, I was dismissed from my job. So now I'm currently unemployed. BUT THEN today I got a call to come in for an interview for basically my dream job (I almost cried I was so excited) and I am really hoping I get this job. After calming down a bit I started to wonder if there was anyway they still might let me go on vacation if I get this job? Of course, I know it would be unpaid if they let me go and if they tell me there is no way I could still go on vacation I would just accept that and be ok with it, but has anyone ever gone through this? Has anyone gotten a new job but already planned a vacation? What did you say? How did you ask? What did they say? Your responses are really appreciated.
I would not mention it at the initial interview, but if they offer you a job I would just inform them that you have a vacation planned. I would be very surprised if they didn't let you go, so long as they know they will plan around it - if they really want you. BTW, this happens all the time. They will get it. Good luck.

Yes, been there. After I had the job I told them that I had a vacation planned from xdate to x date. It wasn't an issue. Congrats and don't sweat it!

When I accepted my current job, I told them I had a vacation planned and the days and they said "no problem!"

If you get a job offer, that is when you let them know the dates of your planned vacation. I did this when I started my job and I am glad I did because my vacation was over a holiday, and where I work we are not allowed to take time off during the holidays (which is really dumb because census is always low then).