H.M.
I just went through this. I didn't mention anything until I was in final interview stages - and at that point it was more 'casual conversation' - we were at a working lunch and I mentioned it off hand.
When I was offered the job (first verbal then via email) I responded, accepted and mentioned my vacation at that point - they were fine with it. Most places are ok with it as long as you aren't particularly demanding about it.
I don't know if I'll get paid or not (probablly not) but like you the vacation has been planned for months and is already paid - no backing out now.
Plus - you don't want to work for a company that would give you a hard time about something like this - it's a good filter!
Good luck