Never on credit cards! For our last big trip I set aside a specific amount per paycheck that went directly into my ING account, plus I used my two "extra" paychecks a year towards it, plus part of my year-end bonus.
This trip is five days (three at Disney, two near the Orlando Convention Center) was a bit different because DD18 paid for part of her share. Her $450 went towards our plane tickets and the hotel. I received some unused sick time paid out, that paid our deposit at Disney. My summer bonus paid off our disney reservation and paid for our Halloween party tickets.
All that's left now is the hotel near the convention center (the event is working out a hotel contract with a good discount, so we are waiting to get details on that) and the two $50 tickets to the event we're attending. I'll pay that out of my normal everyday "spending money" over the next month or two. Since this was a last minute trip I'm just cutting back on frivolous things until then.
We don't have a dedicated vacation fund because we don't always take a trip that needs one. I want to do the Princess half-marathon in 2016 but I also want to take a full week in 2015, so at some point soon I'll start figuring out how much I need to start saving per week.