As a hiring manager in real life, I would appreciate being asked by the potential employee at the time of the offer - as long as it is approached correctly. Demanding the time off is way different from asking & seeming willing to fore-go it if necessary. Plus, there's a difference in the timing of the vacation - 6 months down the road is very different from 6 weeks into the new employee training period.
If you don't come across as a flexible employee at the start (being forceful about the preplanned vacation) then it could surely give me pause about hiring you in the first place.
However, if you ask me about a preplanned vacation and offer to change or cancel it upfront if I prefer, than I am more likely to have no problem with it at all if scheduling permits.
Asking me during the interview process makes it too easy for me to give preference to other qualified candidates. Simply put, if two equal candidates are in front of me, it's a factor that could tip the scale in the other persons favor.
Of course, in my case, if you tell me you're going to Disney World, it's a no brainer that I'd let you go! I might even want to go with you!
